摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX021

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX023

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 84(6)1751-2214 (1988) 1751 1773 1779 1795 807 817 835 847 1853 1863 1871 1879 1889 1897 191 1 1919 1927 1941 1949 1961 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Cooperative Effects in Heterogeneous Catalysis. Part 2.-Analysis and Modelling of the Temperature Dependence of the Oscillating Catalytic Oxidation of CO on a Palladium Al,O,-supported Catalyst P. J. Plath, K. Moller and N. I. Jaeger L-Edge EXAFS Studies of the Coordination of Lead in PbO-PbF, Glasses B. G. Rao, K. J. Rao and J. Wong Investigation of the Coordination of Lead in PbO-PbF, Glasses using XANES K. J. Rao, B. G. Rao and J. Wong The Dimer State of NO in Micropores of &(OH),-dispersed Activated Carbon Fibres K. Kaneko, A.Kobayashi, T. Suzuki, S. Ozeki, K. Kakei, N. Kosugi and H. Kuroda The Association and Solvation of Formamide in Pyridine and Picolines P. P. Singh Analysis of Temperature-programmed Diffusion Chromatograms obtained with Zeolite-Gas Systems The Behaviour of Encapsulated Non-polar Gases in Cs,Na-A Zeolites D. Fraenkel, B. Ittah and M. Levy Solvatochromatic Indicator Study of Silicalite and Zeolite ZSM-5 G. P. Handreck and T. D. Smith Prediction of Excess Volumes of Ternary Liquid Mixtures J. D. Pandey, R. K. Shukla, A. K. Shukla and R. D. Rai Characterization of Ni-exchanged M ontmorilloni tes by X- Ray Photoelectron Spectroscopy M. Stocker, K.-J. Jens, T. Riis and J. K. Grepstad Inhibition of the Thin-film Oxidation of n-Dodecane by Diphenylamine A. D. Ekechukwu and R.F. Simmons Thermodynamics of the Zinc Sulphide Transformation, Sphalerite 3 Wurtzite, by Modified Entrainment P. J. Gardner and P. Pang Pairwise Interaction Parameters for Sodium, Potassium and Halide Ions in Aqueous Solutions A. C. R. Antonini, M. J. Blandamer, J. Burgess, A. W. Hakin, N. D. Hall and A. H. Blandamer Activity Measurements and Spectroscopic Studies of the Catalytic Oxidation of Toluene over Silica-supported Vanadium Oxides B. Jonson, B. Rebenstorf, R. Larsson and S. L. T. Andersson A General Calculation of Molecular Solvation Energies R. J. Abraham, B. D. Hudson, M. W. Kermode and J. R. Mines Excess Enthalpies of Ternary Aqueous Solutions of Amides and Ureas at 298.15 K G. Barone, G. Castronuovo, P. Del Vecchio and V. Elia The Enthalpies of Interaction of some Amides with Urea in Water at 25 "C P.J. Cheek and T. H. Lilley Adsorption of Water and Methanol on Colloidal Iron(II1) Oxide Hydroxides by Infrared Spectroscopy T. Ishikawa, H. Sakaiya and S. Kondo The Effect of Pressure on the 'Crystal-like' Ordering of Monodisperse Polystyrene Spheres in Deionized Aqueous Suspension T. Okubo Solute Orientation Effects in Adsorption Liquid Chromatography M. Borowko D. Fraenkel and A. Levy 58 FAR 1Contents A Comparison of Experimental and Theoretical Adsorption Kinetics of Dichloromethane Vapour by Active Carbon under Non-isothermal Conditions F. Meunier, L.-M. Sun, F. Kraehenbuehl and F. Stoeckli The Temperature Variation of the Hydrophobic Effect M. H. Abraham and E. Matteoli Infrared Study of Ammonia and Nitric Oxide Adsorption on Silica-supported Iron Catalysts C.Johnston, N. Jorgensen and C. H. Rochester Rotating-disc Electrode Voltammetry. The Catalytic Mechanism (EC’) and its Nuances R. G. Compton, M. J. Day, M. E. Laing, R. J. Northing, J. I. Penman and A. M. Waller Effect of Acidity on the Reactivity of the Triplet State of 2-Nitrothiophen L. J. A. Martins and T. J. Kemp Catalytic Properties of the Aluminium Form of Zeolite Y modified with Trifluoromethane S. Kowalak Thermodynamics of Water Sorption by Perfluorosulphonate (Nafion- 1 17) and Polystyrene-Divinylbenzene Sulphonate (Dowex 50W) Ion-exchange Resins at 298f 1 K K. K. Pushpa, D. Nandan and R. M. Iyer Rotating-disc Electrodes. Single- and Double-potential Step Chronoam- perometry and the ECE-DISP1 Problem R.G. Compton, D. Mason and P. R. Unwin Influence of Salts on Poly(Methy1 Vinyl Ether) at the Air/Aqueous Solution Interface. Part 2.-Adsorption from Solution D. D. Eley, M. J. Hey and J. M. Speight Calorimetric Investigations of Solutions of CsI, NaBPh, and Ph,PCl in the HMPT-Water System at 298.15 K The Binding of Sodium Hexadecyl Sulphate to Poly(N-vinylpyrrolidone) in Aqueous Solutions. Kinetic and Equilibrium Studies D. M. Painter, D. M. Bloor, N. Takisawa, D. G. Hall and E. Wyn-Jones Carbon Monoxide Oxidation Kinetics on Zinc Oxide M. Kobayashi, T. Kanno and T. Kimura The Dehydrogenation of Ethanol in Dilute Aqueous Solution Photosensitised by Benzophenones P. Green, W. A. Green, A. Harriman, M.-C. Richoux and P. Neta Adsorption of Nitrogen Monoxide on Iron Oxides supported on Various Supports and its Carrier Effects H.Miyata, Y. Nakagawa, S. Miyagawa and Y. Kubokawa Radiotracer Studies of Chemisorption on Copper-based Catalysts. Part 2.-Adsorption of Carbon Monoxide, Carbon Dioxide and Dihydrogen on Partially and Fully Oxidised Copper-Zinc Oxide-Alumina Catalysts S. Kinnaird, G. Webb and G. C. Chinchen The Water/Oil/Water Thermocouple and the Ionic Seebeck Effect H. H. Girault Mass Transport in Channel Electrodes. The Application of the Backwards Implicit Method to Electrode Reactions (EC, ECE and DISP) involving Coupled Homogeneous Kinetics R. G. Compton, M. B. G. Pilkington and G. M. Stearn Structure Dependence in the Hydrogenation of Diolefins over Ru Thin Films J. Tamaki, T. Miyanaga, T. Imanaka and T. Yamane Solvation of Cyanoalkanes [CH,CN and (CH,),CCN]. An Infrared and Nuclear Magnetic Resonance Study G. Eaton, A. S. Pena-Nuiiez and M. C. R. Symons Spectroscopic Investigation of the Interaction of Co,(CO), with MgO and SiO, K. M. Rao, G. Spoto, E. Guglielminotti and A. Zecchina S. Taniewska-Osinska and M. J6iwiak 1973 1985 200 1 2013 2027 203 5 2047 2057 2069 2077 2087 2099 2109 2129 2135 2147 2155 2173 2181 2195Con tents 2209 Pulse Radiolysis Study of Salt Effects on Reactions of Aromatic Radical Cations with C1-. Part 2.-Spectral Shifts and Decay Kinetics of Diphenyl- polyene Radical Cations in the Presence of Tetrabutylammonium Hexafluorophosphate Y. Yamamoto, T. Aoyama and K. Hayashi 58-2Con tents 2209 Pulse Radiolysis Study of Salt Effects on Reactions of Aromatic Radical Cations with C1-. Part 2.-Spectral Shifts and Decay Kinetics of Diphenyl- polyene Radical Cations in the Presence of Tetrabutylammonium Hexafluorophosphate Y. Yamamoto, T. Aoyama and K. Hayashi 58-2

ISSN:0300-9599    

DOI:10.1039/F198884FP075

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

599 61 1 633 643 655 67 1 679 693 705 727 737 745 763 769 775 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions ll,lssue6,1988 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions 11, Issue 6, is reproduced below HF Autorelaxation in u = 1 and u = 2 L. A. Bollati, E. A. Arguello and E. H. Staricco Inhomogeneous Coulomb Fluid. A Functional Integral Approach R. Podgornik and B. Zeks Solvent Effects on the Theta Temperature of Polymers of Various Architectures M. K. Kosmas Effects of Promotion on the Photophysical Properties of Pyrridyl- and Dimethylamino-diphenylhexatrione Derivatives I. D. Johnson, E. W. Thomas and R. B. Cundall Two-photon Dhotodissociation of Jet-cooled NO, near 450 Rotational Distributions of NO M.R. S. McCoustra and J. Pfab Temperature dependence of the Dielectric Relaxation Processes in Glass- forming Materials Monte Carlo Simulation of an 18.45 mol % Aqueous Ammonia Solution Y. Tanabe and B. M. Rode The Cu+-H,O Interaction Potential and its Application to the Study of [Cu(H,O),]+ Clusters at Different Temperatures M. N. D. S. Cordiero, J. A. N. F. Gomes, A. Gonzalez-Lafont, J. M. Huch, A. Oliva and J. Bertran Transport Coefficients of Model Simple Liquids. A Molecular-dynamics Study and Effective Hard-sphere Analysis K. D. Hammonds and D. M. He yes Low-temperature Spectroscopic Measurements of the ' Dimol ' Transitions of Singlet Molecular Oxygen [O,(a lAg) A. P. Billington, P. Borrell and N. H. Rich A Simple Model for Hopf Bifurcations and other Transition Phenomena in Isothermal Homogeneous and Open Chemical Systems R-S.Li Heat Release and Radical Recombination in Premixed Fuel-lean Flames of H,O + 0, + N,. Rate Constants for H + OH + M -+ H,O + M and HO, +OH+H,O+O, Effects of Flexibility on the Dynamics of Ligand Binding. Time Course of Action of Muscarinic Antagonists A. Miklavc, D. Kocjan, E. Avbelj and D. Hadii Coordination Structure of Vanadium(1v) Ions in the Mixed-solvent Systems of Formide and Ammonium Formate M.Miyake, S.Nagahara and T. Suzuki The Structure of Aggregates formed during the very Early Stages of Colloidal Coagulation E. Dickinson and C. Elvington S. S. N. Murthy J. M. Goodings and A. N. Hayhurst (0711 112 71 1923 712000 712145 7/22 18 7/22 19 712254 712265 81016 8/058 8/059 8/060 81072 8/ 132 81167 81225 81252 81309 81328 The following papers were accepted for publication in Faraday Transactions 1 during March, 1988.Viscometric Investigations of NaI Solutions in Water with Tetrahydrofuran S. Taniewska-Osinska and B. Nowicka Studies of Architecture of MOO,-TiO, Catalysts T. Machej, B. Doumain, B. Yasse and B. Delmon Raman and I. R. Spectroscopy of the AlCl,SOCl, System P. A. Mosier- Boss, R. D. Ross, C. J. Gabriel, S. Szpak, J. J. Smith and R. J. Nowak Transference Number Measurements of Silver Nitrate in Pure and Mixed Solvents using the E.M.F. Method Excess Enthalpies and Cross-term Second Virial Coefficients for Mixtures containing Water Vapour Excess Molar Enthalpies of [xH,O+(l -x)C,H,,] (8) up to 698.2 K and 12.0 MPa Reversible and Steady Photogeneration of 4,4’-Bipyridinium Cation Radicals via Excitation of Ion-pair Charge-transfer Complex between 4,4’-Bipyri- dinium and Tetrakis [3,5-bis (trifluoromethyl)phenyl]borate in Organic Solu- tions T.Nagamura and K. Sakai Thermodynamic Parameters for the Complexation Process between Metal(1) Cations and Dibenzocryptand 222 in Dipolar Aprotic Solvents. Linear Correlation between Entropies of Complexation and Entropies of Solvation of Cations Heat Capacity and Corresponding States in Alkan- 1 -01-Normal Alkane Systems An Aluminium-27 Nuclear Magnetic Resonance Study of Ligand Exchange. Kinetic and Equilibrium Properties The Diffusion Coefficients and Effective Charge Numbers of Lignosulphonate. The Influence of Temperature A.K. Kontturi Determination of Diffusion Coefficients and Effective Charge Numbers of Lignosulphonate. The Influence of Ionic Strength and the Valency of the Counter-ion A. K. Kontturi Excess Enthalpies of [x(CH,),CO + (1 - x) C,H,,] in the Supercritical Region C. J. Wormald, N. Al-Bizreh and T. K. Yerlett Excess Heat Capacities and excess Volumes of Normal Alkane Mixtures D. Apam-Martinez and A. Trejo Temperature-programmed Desorption of p-Xylene from ZSM-5, ZSM- 1 1 and Theta-1 Spectral Spin Diffusion in Polycrystalline Solids under Magic Angle Spinning Mechanism of Deuterium Addition and Exchange of Propene over Silica- supported Au and Ag Catalysts Quadrupole Nutation N.M.R. in Solids Coupling of Free Energies in the Formation of Intermediates during the Catalytic Decomposition of H,O, D.S. Gill and M. Singh Bakshi C. J. Wormald and N. M. Lancaster N. M. Lancaster and C. J. Wormald A. F. Danil de Namor and F. F. Salazar L. Andreoli-Ball, D. Paterson and M. C. M. Caceres-Alonso T. Jin and K. Ichikawa I. F. Chen and L. V. C. Rees A. Kubo and C. A. McDowell s. Naito and M. Tanimoto R. Janssen and W. S. Veeman M. L. Kremer (ii)8/361 8/415 8/453 8/454 8/509 8/510 8/51 1 8/512 Nature of Acid Sites in SAP05 Molecular Sieves Part 1.-Effects of the Concentration of Silicon incorporated and of Amorphous Impurities C. Halik, J. A. Lercher and H. Mayer Carbon- 13 Chemical Shift Tensors in Single-crystal Methoxybenzenes C. M. Carter, J. C. Facelli, N. Kent Dalley, B. E. Wilson, D. W. Alderman and D. M. Grant Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300 "C.Part 1.-X-Ray Photoelectron Spectroscopy G. C. Allen, P. A. Tempest and J. W. Tyler Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300 "C. Part 2.-X-Ray Diffraction and Scanning Electron Microscopy G. C. Allen, P. A. Tempest and J. W. Tyler Perspectives in High-resolution Solid-state N.M.R., with Emphasis on Combined Rotation and Multiple-pulse Spectroscopy R. K. Harris, P. Jack- son, L. H. Merwin, G. J. Nesbitt, B. J. Say and G. Hagele E.S.R. Studies of the Reactions of Aluminum Atoms with some Alkenes in a Rotating Cryostat Generation and Reactions of the Chlorine Atom in Aqueous Solution B. C. Gilbert, J. K. Stell, and (in part) W. J. Peet and K. J. Radford Spin Trapping with Thiocarbonyl Compounds A. Alberti, M.Benaglia, B. F. Donini and G. F. Pedulli J. A. Howard, H. A. Joly, B. Mile and M. Histed (iii)8/361 8/415 8/453 8/454 8/509 8/510 8/51 1 8/512 Nature of Acid Sites in SAP05 Molecular Sieves Part 1.-Effects of the Concentration of Silicon incorporated and of Amorphous Impurities C. Halik, J. A. Lercher and H. Mayer Carbon- 13 Chemical Shift Tensors in Single-crystal Methoxybenzenes C. M. Carter, J. C. Facelli, N. Kent Dalley, B. E. Wilson, D. W. Alderman and D. M. Grant Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300 "C. Part 1.-X-Ray Photoelectron Spectroscopy G. C. Allen, P. A. Tempest and J. W. Tyler Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300 "C.Part 2.-X-Ray Diffraction and Scanning Electron Microscopy G. C. Allen, P. A. Tempest and J. W. Tyler Perspectives in High-resolution Solid-state N.M.R., with Emphasis on Combined Rotation and Multiple-pulse Spectroscopy R. K. Harris, P. Jack- son, L. H. Merwin, G. J. Nesbitt, B. J. Say and G. Hagele E.S.R. Studies of the Reactions of Aluminum Atoms with some Alkenes in a Rotating Cryostat Generation and Reactions of the Chlorine Atom in Aqueous Solution B. C. Gilbert, J. K. Stell, and (in part) W. J. Peet and K. J. Radford Spin Trapping with Thiocarbonyl Compounds A. Alberti, M. Benaglia, B. F. Donini and G. F. Pedulli J. A. Howard, H. A. Joly, B. Mile and M. Histed (iii)Cumulative Author Index 1988 Abe, H., 511 Abraham, M. H., 175, 865, 1985 Abraham, R.J., 1911 Adachi, H., 1091 Aicart, E., 1603 Allen, G. C., 165, 355 Amorelli, A., 1723 Anazawa, I., 275 Andersson, S. L. T., 1897 Anpo, M., 751 Antonini, A. C. R., 1889 Aoyama, T., 2209 Aracil, J., 539 Arora, K. S., 1729 Asakura, K., 1329 Aveyard, R., 675 Baba, K., 459 Bagchi, S., 1501 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Barone, G., 1919 Baulch, D. L., 1575 Bazsa, G., 215, 229 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Bertoldi, M., 1405 Beyer, H. K., 1447 Binks, B. P., 675 Blandamer, A. H., 1889 Blandamer, M. J., 1243, 1889 Blesa, M. A., 9 Blinov, N. N., 1075 Bloor, D. M., 2087 Bonnefoy, J., 941 Borbdy, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Borowko, M., 1961 Bourdillon, C., 941 Brandreth, B.J., 1741 Breen, J., 293 Briggs, B., 1243 Brown, M. E., 57, 1349 Brydson, R., 617, 631 Burgess, J., 1243, 1889 Burget, U., 885 Busca, G., 237, 1405, 1423 Buxton, G. V., 1101, 1113 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone, A. I., 207 Carr, N. J., 1357 Castronuovo, G., 1919 Cavani, F., 237 Cavasino, F. P., 207 Centi, G., 237 Chagas, A. P., 1065 Chandra, H., 609 Che, M., 751 Cheek, P. J., 1927 Cheng, V. K. W., 899 Chien, J. C. W., 1123 Chinchen, G. C., 2135 Chirico, G., 979 Chudek, J. A., 1145, 1737 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483, 2013, 2057, 2155 Cook, A., 1691 Costas, M., 1603 Covington, A. K., 1393 Crowther, N. J., 1211 Danil de Namor, A. F., 255, Das, S ., 1057 Dash, A. C., 75 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 Day, M. J., 2013 de Bleijser, J., 293 Del Vecchio, P., 1919 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dyster, S., 11 13 Eagland, D., 121 1 Eaton, G., 2181 Egawa, C., 321 Einfeldt, J., 93 1 Ekechukwu, A. D., 1871 Eley, D. D., 2069 Elia, V., 1919 Elliot, A. J., 1101 Engel, W., 617, 631 Eszterle, M., 575 Evans, J. C., 1723 Everett, D. H., 1455 Eyears, J. M., 1437 1671 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Flanagan, T. B., 459 Fletcher, P. D. I., 113 1 Foresti, E., 237 Foresti, M. L., 97 Forster, H., 491 Foster, R., 1145, 1737 Fraenkel, D., 1817, 1835 Franklin, K.R., 687 Fubini, B., 1405 Fiiredi-Milhofer, H., 1301 Gal, D., 1075 Gabrail, S., 41 Galwey, A. K., 57, 729, 1349, Gans, P., 657 Gardner, P. J., 1879 Geblewicz, G., 561 Geertsen, S., 1101 Georges, V., 1531 Giamello, E., 1405 Gill, D. S., 1729 Gill, J. B., 657 Gilot, B., 801 Girdult, H. H., 2147 Gopalakrishnan, R., 365 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A. C., 1509 Gray, P., 993 Green, P., 2109 Green, S. I. E., 41 Green, W. A., 2109 Grepstad, J. K., 1863 Griffiths, J. F., 1575 Grigo, M., 931 Grimson, M. J., 1563 Gritzner, G., 1047 Grzybkowski, W., 1551 Guardado, P., 1243 Guarini, G. G. T., 331 Guglielminotti, E., 2195 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Hadjiivanov, K., 37 Hakin, A. W., 1889 Hall, D. G., 773, 2087 Hall, N. D., 1889 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Handreck, G.P., 1847 Hanson, G. R., 1475 Harrer, W., 366 Harriman, A., 2109 1357AUTHOR INDEX Kobayashi, H., 1517 Kobayashi, M., 281, 2099 Koda, S., 1267 Kondo, J., 51 I Kondo, S., 1941 Kondo, Y., I I I Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Kosugi, N., 1795 Kowalak, S., 2035 Kraehenbuehl, F., 1973 Krausz, E., 827 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa, Y., 751, 2129 Kumamaru, T., 1679 Kurimura, Y . , 841, 1025 Kuroda, H., 1329, 1795 Kusabayashi, S., 11 1 Kuwabata, S., 1587 Lahy, N., 1475 Laing, M. E., 2013 Lajtar, L., 19 Lambi, J. N., 1 Larsson, R., 1897 Laubry, P., 969 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., I181 Leaist, D. G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I., 229 Levy, A,, 1817 Levy, M., 1835 Lewis, T.J., 1531 Leyendekkers, J. V., 397, 16 Leyte, J. C., 293 Lilley, T. H., 1927 Lincoln, S. F., 365 Lindner, Th., 631 Llewellyn, J. P., 153 1 Logan, S. R., 1259 Lycourghiotis, A., 1593 MacKay, R. L., 1145, 1737 Maezawa, A., 851 Malanga, C., 97 Marcus, Y., 175, 1465 Markovid, M., 1301 Maroto, A. J. G., 9 Martins, L. J. A., 2027 Maruya, K., 511 Mason, D., 473, 483, 2057 Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Matteoli, E., 1985 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mensch, C. T. J., 65 1713 Lips, A., 1223 (vi) Hasebe, T., 187 Hashimoto, K., 87 Hayashi, K., 2209 Hazra, D. K., 1057 Heatley, F., 343 Herley, P. J., 729 Herrmann, J-M., 261 Hey, M. J., 2069 Heyward, M.P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Hubbard, C. D., 1243 Hudson, B. D., 1911 Huis, D., 293 Hunter, R., 1311 Hutchings, G. J., 1311 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Imamura, H., 765 Imanaka, T., 851, 2173 Inoue, A., 1195 Irinyi, G., 1075 Ishikawa, T., 1941 Isobe, T., 1199 Ito, D., 1375 Ittah, B., 1835 Iwamoto, E., 1679 Iwasawa, Y., 321, 1329 Iyer, R. M., 2047 Jackson, S. D., 1741 Jaeger, N. I., 1751 Jaenicke, W., 366 Jeminet, G., 951 Jens, K-J., 1863 Johnson, G. R. A., 501 Johnson, I., 551 Johnston, C., 309, 2001 Jonson, B., 1897 Jorge, R. A., 1065 Jorgensen, N., 309, 2001 Joiwiak, M., 2077 Juillard, J., 951, 959, 969, Kaizu, Y., 1517 Kakei, K., 1795 Kane, H., 851 Kaneko, K., 1795 Kanno, T., 281, 2099 Kasahara, S., 765 Kato, S., 151 Katz, N.E., 9 Kawasaki, Y., 1083 Keeble, D. J., 609 Kemp, T. J., 2027 Kermode, M. W., 1911 Kevan, L., 467 Kimura, T., 2099 Kinnaird, S., 2135 Kirby, C . , 355 Kiricsi, I., 491 Kiss, I., 367 Kiwi, J., I703 Klinszporn, L., 1551 Klissurski, D., 37 Kobayashi, A,, 1795 Merkin, J. H., 993 Meunier, F., 1973 Mills, A., 379, 1691 Mines, J. R., 1911 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miyagawa, S., 2129 Miyakawa, K., 1517 Miyanaga, T., 2173 Miyata, H., 2129 Mohamed, M. A-A., 57, 729, Moiroux, J., 941 Moller, K., 1751 Morris, J. J . , 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mousset, G., 969 Muhler, M., 631 Murray, B. S., 871 Nagao, M., 1277 Nakagawa, Y., 2129 Nakamura, T., 1287 Nakamura, Y., 1 1 1 Nakao, N., 665 Nakayama, N., 665 Nandan, D., 2047 Narayanan, S., 521 Nazhat, N.B., 501 Neta, P., 2109 Newman, K. E., 1387, 1393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nomura, H., 151, 1267 Norris, J. 0. W., 441 Northing, R. J., 2013 Noszticzius, Z . , 575 Nucci, L., 97 Ohshima, K., 1639 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, Y . , 851 Okubo, T., 703, 1163, 1171, Oliver, S. W., 1475 Olofsson, G., 551 Ommen, J. G. van, 1491 Onishi, T., 51 1 Ono, Y., 1091 Oosawa, Y., 197 Ozeki, S., I795 Page, F. M., I145 Painter, D. M., 773, 2087 Pal, M., 1501 Pan, C.-f., 1341 Pandey, J. D., 1853 Pang, P., 1879 Pappin, A. J., 1575 Parrott, D., 1131 Passelaigue, E., 17 13 Patterson, D., 1603 1349 '53 1949Pelizzetti, E., 261 Pena-Nuiiez, A. S., 2181 Penar, J., 739 Penman, J.I., 2013 Pezzatini, G., 367 Piccini, S., 331 Pichat, P., 261 Pickl, W., 131 1 Piekarski, H., 529, 591 Prlarczyk, M., 1551 Pilbrow, J. R., 1475 Pilkington, M. B. G., 2155 Plath, P. J., 1751 Pointud, Y., 959, 1713 P6ta, G., 215 Preston, K. F., 413 Prior, D. V., 865 Pushpa, K. K., 2047 Quist, P-O., 1033 Radulovic, S., 1243 Rai, R. D., 1853 Rajam, S., I349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rebenstorf, B., 1897 Rtmuncio, J. A. R., 539 Rhodes, C. J., 1187 Richoux, M-C., 2109 Riis, T., 1863 Riva, A., 1423 Rochester, C. H., 309, 2001 Rojas, F., 785, 801, 1455 Ross, J. R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saito, M., 1025 Saito, Y., 275 Sakaiya, H., 1941 Sakamoto, Y., 459 Sakata, Y., 511 Salvagno, S., 1531 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Schelly, Z.A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schulz. R. A., 865 Riio, K. M., 2195 AUTHOR INDEX Schwarz, W., 1703 Scott, S. K., 993 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P. A., 391 Serpone, N., 261 Shindo, H., 433 Shukla, A. K., 1853 Shukla, R. K., 1853 Sidahmed, I. M., I153 Simmons, R. F., 1871 Sinclair, G. R., 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Smith, E. R., 899 Smith, T. D., 1475, 1847 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J. M., 2069 Spoto, G., 2195 Stainsby, G., 871 Stearn, G. M., 2155 Stevens, J. C. H., 165 Stirling, C.J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, W. E. E., 117 Stramel, R. D., 1287 Subba Rao, M., 1703 Sun, L-M., 1973 Suzuki, T., 1795 Sykes, A. F., 1575 Symons, M. C. R., 609, I18 1, 1187, 2181 Szamosi, J., 917 Taga, K., 1639 Tagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takato, K., 841 Takisawa, N., 2087 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K.. 601 Tanaka, K-i., 601 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D. M., 1531 Taylor, P. J., 865 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631 Tissier, C., 951, 969 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 Townsend, R. P., 687 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tsuchiya, S., 765 Tsukamoto, K., 1639 Twiselton, D.R., 1145 Uematsu, R., I 1 1 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Vaccari, A., 1405, 1423 van Rensburg, L. J., 131 1 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013 Ward, J., 713 Webb, G., 2135 Wells, C. F., 815, 1153 Welsh, M. R., 1259 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N. D., 11 13 Wormald, C. J., 1437 Wyn-Jones, E., 773, 2087 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, S., 1083 Yao, S., 1375 Yoneyama, H., 1587 Yoshida, S., 87 Zecchina, A., 751, 2195 Zeitler, E., 617, 631 Zelano, V., 29 Zielinski, R., 151 Zundel, G., 885 647 (vii)Pelizzetti, E., 261 Pena-Nuiiez, A.S., 2181 Penar, J., 739 Penman, J. I., 2013 Pezzatini, G., 367 Piccini, S., 331 Pichat, P., 261 Pickl, W., 131 1 Piekarski, H., 529, 591 Prlarczyk, M., 1551 Pilbrow, J. R., 1475 Pilkington, M. B. G., 2155 Plath, P. J., 1751 Pointud, Y., 959, 1713 P6ta, G., 215 Preston, K. F., 413 Prior, D. V., 865 Pushpa, K. K., 2047 Quist, P-O., 1033 Radulovic, S., 1243 Rai, R. D., 1853 Rajam, S., I349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rebenstorf, B., 1897 Rtmuncio, J. A. R., 539 Rhodes, C. J., 1187 Richoux, M-C., 2109 Riis, T., 1863 Riva, A., 1423 Rochester, C. H., 309, 2001 Rojas, F., 785, 801, 1455 Ross, J.R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saito, M., 1025 Saito, Y., 275 Sakaiya, H., 1941 Sakamoto, Y., 459 Sakata, Y., 511 Salvagno, S., 1531 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schulz. R. A., 865 Riio, K. M., 2195 AUTHOR INDEX Schwarz, W., 1703 Scott, S. K., 993 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P. A., 391 Serpone, N., 261 Shindo, H., 433 Shukla, A. K., 1853 Shukla, R. K., 1853 Sidahmed, I. M., I153 Simmons, R. F., 1871 Sinclair, G. R., 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Smith, E. R., 899 Smith, T.D., 1475, 1847 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J. M., 2069 Spoto, G., 2195 Stainsby, G., 871 Stearn, G. M., 2155 Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, W. E. E., 117 Stramel, R. D., 1287 Subba Rao, M., 1703 Sun, L-M., 1973 Suzuki, T., 1795 Sykes, A. F., 1575 Symons, M. C. R., 609, I18 1, 1187, 2181 Szamosi, J., 917 Taga, K., 1639 Tagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takato, K., 841 Takisawa, N., 2087 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K.. 601 Tanaka, K-i., 601 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D. M., 1531 Taylor, P. J., 865 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631 Tissier, C., 951, 969 Tofield, B.C., 441 Torres-Sanchez, R-M., 117 Townsend, R. P., 687 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tsuchiya, S., 765 Tsukamoto, K., 1639 Twiselton, D. R., 1145 Uematsu, R., I 1 1 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Vaccari, A., 1405, 1423 van Rensburg, L. J., 131 1 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013 Ward, J., 713 Webb, G., 2135 Wells, C. F., 815, 1153 Welsh, M. R., 1259 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N. D., 11 13 Wormald, C.J., 1437 Wyn-Jones, E., 773, 2087 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, S., 1083 Yao, S., 1375 Yoneyama, H., 1587 Yoshida, S., 87 Zecchina, A., 751, 2195 Zeitler, E., 617, 631 Zelano, V., 29 Zielinski, R., 151 Zundel, G., 885 647 (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION N o . 86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. 6. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments.The Introductory Lecture will be given by G. C. Pimentel and speakers include: L. Andrews, K. H. Bowen, 6. J. Howard, L. 6. Knight Jr, E. Knozinger, D. H. Levy, J. P. Maier, J. Michl, M. Moskovits, A. J. Stace, M. Takami, J. J. Turner, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Symons and P. Suppan. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN ~~~ THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE I T A L I A N A D I CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCIETE FRANCAISE DE CHlMlE A N D DEUTSCHE J O I N T MEETING BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee: M.Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physical methods: (i) Metals (both in single crystal and dispersed form) (ii) Insulators and semiconductors (oxides, sulphides, halides, both in single crystal and dispersed forms) (iii) Mixed systems (with special emphasis on metal-support interaction) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods.Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Fisica, Corso Massimo D'Azeglio 48, 10125 Torino, Italy.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S. Stolte Professor J. P. Simons Dr K. Burnett Dr H. Loesch Professor R. N. Dixon Professor R. A. Levine The Symposium will focus on the study of vector properties in reaction dynamics and photodissociation rather than the more traditional scalar quantities such as energy disposal, integral cross-sections and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interactions.The Symposium will provide an impetus to the development of 3-D theories of reaction dynamics and assess the quality and scope of the experiments that are providing this impetus. Contributions for consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ‘collisions’ in van der Waals complexes (D) Polarisation of the products of full and half-collisional processes The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 87 Catalysis by Well Characterised Materials University of Liverpool, 11-13 April 1989 Orga nising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone Dr. K. C. Waugh Professor P. B. Wells The understanding of heterogeneous catalysis is an important academic activity, which compliments industry‘s continuing search for novel and more efficient catalytic processes. The emergence of relevant, in particular in situ techniques and new developments of well established experimental approaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships. Well characterised catalysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and appropriate industrial catalysts.The elucidation of structure/function relationships and catalytic mechanism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisation techniques will also be welcome. Further information may be obtained from: Professor R. W. Joyner, Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Grove Street, P. 0.Box 147, Liverpool L69 3BX.FARADAY DIVISION INFORMAL AND GROUP MEETINGS Neutron Scattering Group Postgraduate Informal Neutron Conference To be held at the University of Keele on 11-13 July 1988 Further information from Professor C. R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 5BG Colloid and Interface Science Group with the Biochemical Society Dynamic Properties of Biomolecular Assemblies To be held at the University of Nottingham on 20-22 July 1988 Further information from Dr S. E. Harding, School of Agriculture, Unversity of Nottingham, Department of Applied Biochemistry, Sutton Bonington LE12 5RD Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G.Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 Electrochemistry Group with the Electroanalytical Group and the Society of Chemical Industry Electrochemcial Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held at the University of Cambridge on 14-16 September 1988 Further information from Dr P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of the SCI Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SWlX 8PS Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I. W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birmingham B15 2TT Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI pic, Corporate and Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Ph ysics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Society, London on 30 November 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex W 1 1 OLWPolar Solids Group with the Applied Solid State Chemistry Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies’ Lecture Theatre, London on 2 December 1988 Further information from Dr A. E. Comyns, R & D Department, Laporte Industries Ltd., Moorfield Road, Widnes WA8 OQJ Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies’ Lecture Theatre, London on 16 December 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, P.O. Box 11, The Heath, Runcorn, Cheshire WA7 4QE Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP079

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chern. SOC., Faraday Trans. I , 1988, 84(6), 1751-1771 Cooperative Effects in Heterogeneous Catalysis Part 2.-Analysis and Modelling of the Temperature Dependence of the Oscillating Catalytic Oxidation of CO on a Palladium A120,-supported Catalyst Peter J. Plath,* Karin Moller and Nils I. Jaeger Institut fur Angewandte und Physikalische Chemie, Universitat Bremen, Bibliothekstrasse, N W 2, 0-2800 Bremen 33, Federal Republic of Germany The temperature dependence of the dynamics of the heterogeneous oxidation of CO on palladium supported by an amorphous A1,0, carrier has been studied. The complex structure of the observed time series has been investigated by fast-Fourier-transform analysis. The resulting spectra are characterized by at most three frequencies, the ratio of which is nearly an integer, and by frequency locking.For lower temperatures sub-harmonics of these frequencies become important. The fractal character of the time series could be explained qualitatively by a one-dimensional automaton rep- resenting a concentration wave travelling on a fractal network of Pd particles in the catalyst bed. The complex phenomenology of the dynamics of CO oxidation on palladium embedded in a zeolite matrix has been presented in detail in ref. (1). The interpretation of the observed oscillations was based on the assumption of phase transitions between a catalytically active Pd-metal phase and an inactive palladium oxide phase. The idea of phase transitions involving the catalyst has been proposed by Wagner2 and was used in early models put forward to account for observed reaction rate oscillation^.^-^ Phase transitions have since been observed in situ on supported Pd catalyst' and on single-crystal surfaces of Pt.' A mathematical model of ideal storage' involving the phase transition of the bulk of catalytically active crystalli tes showed important dynamic properties observed in experimental systems without considering the necessary coupling of active palladium particles.This model represents a fully synchronized catalyst and might be valid only under special experimental conditions. Strong coupling between the catalytically active parts of the catalyst is needed to fulfil the conditions of this model, For instance, thermal coupling of neighbouring catalytic areas undergoing phase transitions has been used successfully to simulate the dynamics of the oxidation of methanol on a supported Pd ~atalyst.~-ll However, the phenomenology of catalytic CO oxidation is of much higher complexity than that of methanol ~ x i d a t i o n ' * ~ * ~ ~ , l3 because of the smaller coupling between the Pd particles within the catalyst.Again using the idea of chemical stores undergoing phase transitions, the temporal pattern of this reaction could be described qualitatively by a linear deterministic cellular automaton. 14* l5 This automaton models the temporal development of the number of catalytic centres, which are blocked for the conversion of CO at a given time. Even though this model creates a discrete temporal pattern, which is very similar to the experimental time series, a detailed analysis of these time series has not been made.It is our aim now to analyse the time series of catalytic CO oxidation by Fourier analysis in order to obtain a deeper insight into this reaction. 17511152 Oscillating Catalytic Oxidation of CO Theoretical Background The short temporal decrease of the conversion requires the synchronization of very large numbers of Pd crystallites. Synergistic effects concerning the chemical behaviour of Pd particles located far from each other on a microscopic scale should therefore be taken into account. Synchronized areas may be formed by agglomerates of zeolite crystals or even by single crystals,' or by regions of sufficiently closed packed amorphous material in the case of Pd/A1,0, catalyst.Consider the synchronized domains as new active units, where all Pd crystals undergo phase transitions between catalytic active Pd and inactive PdO at the same time. We now have a set of more-or-less randomly distributed reactive units among which coupling should occur, mainly via diffusion of CO and 0,. This leads to a system of loosely coupled reactive units, each of which can be regarded as a bistable oscillator with respect to the Pd/PdO phase transition. Such a system may form an excitable medium of discrete nature and the number of leading centres working as oscillators will depend on the amount and packing of the catalyst. The chemical excitation, which starts from such an unstable leading centre, will spread out with time and might dominate macroscopically large parts of the catalyst.It has to be emphasized that it is the chemical excitation which is spreading out and not the concentration of the reactants. The nature of this excitation is assumed to be the phase transition of the reactive units from the chemically active Pd to the inactive PdO state. The functions of the time dependence of the phase transition itself cannot be specified at the present time. Furthermore, we do not have any detailed knowledge about the spatial distribution of the reactive units within the catalyst. The reactive units may form a fractal or a regular lattice and the excitation may travel as a spherical wave along the fractal arrangement of the reactive units, or the excitation itself may form a fractal in the latter case. An approach to understand and describe the dynamics of this process is suggested by the self-similar pattern of the time series of the conversion (see fig.1). This temporal behaviour could be described by a model, having the intrinsic feature of self-similarity . To simplify our system, first we restrict ourselves to one dimension, and secondly we take a discrete arrangement of cells as a valuable model, since our process acts on a set of distinguishable reactive units. The state of each cell now represents the phase of the reactive units. Thirdly, we introduce a discrete time to model the discrete character of the events, i.e. the abrupt decrease of the conversion. These events are correlated to the phase transition of the reactive units, and we have to introduce a transition rule for the states of the cells.The state of a cell at time (n + 1) depends on its state at time n and also on the state of some neighbouring cells at time n. With this set of assumptions a one- dimensional cellular automaton can be established, the temporal sequence of states of which represents a discrete version of the Sierpinsky gasket, a typical example for self- similar s t r u ~ t u r e s . ' ~ ~ ~ ~ In the framework of this elementary model we now have to explain the periodic behaviour of our real system. Since we have a recurrence of self-similar intervals we can simply model this by a reinjection process of the bounded automaton, starting the same process again from the same initial cell of the automaton. In chemical reality this means that after a given period of time our system reaches its boundary, i.e.under the constraints of the reaction the maximum number of reactive units become inactive. After reactivation, anywhere in the excitable medium one unit becomes active and may develop itself into a leading centre. It is not necessary that the same cell always becomes the leading cell, but in the mathematical model we can identify any of the possible leading centres with the starting cell of the automaton. With this model in mind, the reconstruction of the time series in three-dimensional space and their Fourier spectra will be presented and discussed.P. J . Plath, K . Miiller and N . I. Jaeger 1753 Experimental The reactions were carried out in a differential flow reactor' (volume 16 cm3) at different temperatures To between 434 and 507 K with a CO concentration of 0.37 YO by volume in the feed.The gas mixture was prepared from synthetic air (impurities < 0.1 ppm) and a certified gas mixture containing CO and N, (Messer Griesheim). A molecular sieve was used for additional purification of the gas mixture. Measurements were carried out with 50 mg of the Pd/AI,O, catalyst with 0.5 YO Pd by weight (Heraeus). The average size of the Pd crystallites was less than 2 nm. The CO or CO, concentration in the outlet of the reactor was continuously recorded by i.r. spectroscopy (URAS, Hartmann and Braun). The analogue signal was converted into digital data, stored and evaluated off-line on a PDP-11 computer (Digital Equipment). Prior to starting a catalytic reaction the catalyst was heated in synthetic air up to 600 K (heating rate 5 K min-') and kept at this temperature for 12 h.The catalytic experiments were conducted thereafter at constant CO concentration (0.37 vol YO) and with stepwise variation of the temperature (AT= 2-5 K). The system was kept under constant conditions for at least 22 h after each temperature variation. For further experimental details see ref. (1). Results Fig. 1 shows characteristic patterns of the time series for various temperatures of the reactor. The time series represents sudden breaks in the rate of conversion. With decreasing temperature an increase in frequency as well as in temporal deactivation can be observed. Multistability can be observed at lower temperatures, and eventually the reaction is quenched.In this region between 433443 K [see fig. 1 (B)] small changes in temperature force the system into an inactive state for a finite time, where the duration of the suppression depends on the specific reactor temperature. The exponential increase of the ignition times connected with decreasing the temperature of the reactor, To, is plotted in fig. 2. Aside from different pre-exponential factors, an apparent activation energy between 3 15 and 322 kJ mol-' can be estimated. Taking into account the time of low activity during multistable behaviour one can calculate an apparent activation energy of ca. 479 kJ mol-' even for the autonomous changes of the systems in the range of its multistable situation. A similar activation energy has been observed by Turner et al." obtained from measurements of the induction period of the oscillations. The order of magnitude of these activation energies points to processes which are not compatible with desorption, adsorption or surface reaction. For instance the chemisorption of CO and 0, is not achieved in this temperature range,18 the desorption of CO possesses an activation energy of 117-1 51 kJ mol-l, whereas for oxygen it is ca.180-251 kJm0l-l.l' For the surface diffusion of oxygen 63 kJ rnol-l2O and for CO oxidation on a PdIII surface following the Langmuir- Hinshelwood mechanism, an activation energy between 59-105 kJ mol-' 21 is required. A possible explanation could be given by the assumption that the diffusion of oxygen from the bulk to the surface of an oxidized and inactive palladium crystal requires this high activation energy.This would be in agreement with our previous interpretation' of the observed patterns involving phase transitions between Pd and PdO. The observed temperature dependence of the time needed for the reactivation leads to the conclusion that the PdO phase should be stabilized at lower temperature by shifting the thermodynamic equilibrium towards PdO. Comparable conclusions can be drawn by looking at the stability of surface PdO;,, therefore, one expects an increase of the number of palladium oxide particles for decreasing reaction temperatures. The assumption leads to a stronger decrease of the conversion of CO to CO, due to the more favourable condition to form PdO crystals. On the other hand, taking CO to reduce PdO1754 Oscillating Catalytic Oxidation of CO A 50 60 min t f B I I 60 min t Fig.1. Characteristic self-similar time series. Dependence of the conversion of CO into CO, on the temperatyre To of the reactor. [Pd/Al,O, catalyst (0.5 YO Pd by wt); 50 mg catalyst, Pd particle diameter 20 A, surface area of Pd particles 1.25 m2 (g catalyst)-', 0.37 YO CO by volume.] A (a) T = 223 "C, (b) T = 213 "C, (c) T = 203 "C, ( d ) T = 175 "C; B (0) T = 168 "C, (b) T = 164 "C.P. J . Plath, K. Moller and N . I. Jaeger 1755 I \ \. t I I 160 170 180 190 T0l"C Fig. 2. Ignition times of the reaction after lowering the temperature to the finite reaction temperature for two experiments (a) and (b) with the same amount of the catalyst (50 mg). The inset shows an Arrhenius plot. no formation of CO, could be observed for temperatures less than 403 K, which can be understood by the high activation energy of the reduction of PdO.Analysis of the Time Series To gain deeper insight into the nature of the process underlying the time series, we have investigated their pattern by reconstructing the trajectory of the system in a three- dimensional phase space using the method mentioned ea~1ier.l~' l5> 23 Fig. 3 shows the trajectories of our system at four different temperatures in the range 438-496 K. For the graphic representation we have chosen a time interval of 1 h from the corresponding time series, which are given in fig. 1. The trajectories form attractors, the shape of which reminds us of 'strange attractors' representing chaotic behaviour. 23-29 The characteristic feature of ' strange attractors ' is their sensitive dependence upon the initial conditions.For our experimental investigation this means that the sequence of the orbits of the attractor differs for each interval of the experimental time series, although the topology of its embedding remains constant with time. However, one has to be very careful in drawing conclusions from details of the geometrical shape of the attractors, since the shape depends upon the chosen time interval z used for the construction of the phase space. This can be understood by looking at the attractors resulting from the same time series at 496 K the sequences of the realizations of the two bundles of orbits differ [see fig. 3(A) and 41.At 476 K the system is represented by an attractor, the orbits of which differ extremely and cover a large area of the phase space, although one can recognize several bundles of orbits [see fig. 3(A)]. The appearance of multistability for lower temperatures leads to a complex1756 Oscillating Catalytic Oxidation of CO Z Z X ' Fig. 3. Representation of the time series of fig. 1 by the trajectories of the system in a three- dimensional phase-space constructed by the variables x = x(t), y = x(t - z) and z = x(t - 22) and a generic time interval z = 3 s: A (a) T = 223 "C, (b) T = 203 "C; B (a) T = 168 "C, (b) T = 164 "C.P. J. Plath, K. Moller and N. I. Jaeger 1757 X Z Fig. 4. The projection of the trajectories of the system at T = 223 "C into the (a) two-dimensional and (b) three-dimensional phase-space; z = 8 s.attractor shape [fig. 3(B)], again characterized by a high sensitivity of the system to the initial conditions. For example, there are some orbits which pass the middle state without touching, while others stay at this state before reaching the lower state of conversion. The appearance of bundles of orbits gives rise to the question whether or not hidden periodicities can be observed, which might be due to dominating leading centres in the excitable two-dimensional catalyst bed. In order to study this problem the power ~ p e c t r a ~ ' ? ~ ~ obtained by Fourier analysis (see fig. 5 and 6) of the time series were analysed. Decreasing the temperature from 606 K the first small-amplitude oscillation occurs at 529 K.The birth of very small oscillations at this critical temperature can be characterized by a weak excitation as it is typical for a Hopf bifurcation. The oscillations show a simple periodic pattern [fig. 5(a)-(c)]. At 496 K one can observe oscillations almost of period two [fig. 5(e)]. Thereafter a more or less periodic pattern develops into oscillations with a fractal structure in time [fig. 5(g)-(i)]. One can follow this looking at the Fourier spectra (see fig. 6). They have been taken by analysing an interval of 1024 s and 150 given harmonics. The fundamental frequencies X (i = 1, 2, 3) can be approximated easily by counting the number of given harmonics within the interval up to the frequency of interest and multiplying this number with the factor 0.058, giving the number of oscillations per minute. For higher temperatures between 508 and 501 K a periodic pattern can be recognized within the Fourier spectra.The fundamental frequencies could be interpreted in a first attempt as a main mode followed by several harmonics since they are close to multiples of this lowest frequency. But in the linear representation of a Fourier spectrum harmonics of the fundamental frequency of a circle should not be observable unless patterns occur, which are non-sinusoidal to such a degree that many higher harmonics with strong amplitudes are required. Otherwise, they become visible only in the logarithmic plot. In our case, the occurrence of 'harmonics' already in the linear plot indicates resonance with other fundamental closely neighbouring frequencies.For example, the number of harmonics visible in the linear plot can grow up to six in the case1758 Oscillating Catalytic Oxidation of CO J A CI YP . J . Plath, K. Moller and N . I . Jaeger 1759 i--1760 A h Y Oscillating Catalytic Oxidation of CO i - c YP . J . Plath, K . Moller and N. I. Jaeger 1761 of T = 507. If one compares a linear spectrum of these time series with the linear representation of the squared amplitudes of a sine function one can recognize that for instance the spectrum for 507 K [see fig. 6(a)] is not only constructed by one frequency, f,, and its harmonics but by at least three frequencies f1(6), f2( 13) and f3(19) and their linear combinations Ifi + mf, + nfk. The simple structure of the spectrum is due to the fact that in the limit of the accuracy of the numerical analysis f, = f3/3 and f 3 - f, = f,, f2 = f,f3, 2f3 = 3f2; 4f1 +f3 = 5f1 +.f2.There is a locking of frequencies. At 505 K the lowest frequencyf1(9) grows in comparison to 507 K. The spectrum can again be formed by three frequencies, f1(9), f2(17) and f3(26). At 501 K the spectrum becomes very simple and it seems to be harmonic; however, the amplitudes of the higher harmonics are too large to be formed by only one frequency, f1(12). For this reason a second and third frequency have to be taken into account again. With decreasing temperature the fundamental frequencies are shifted to higher values. At 498 K bifurcation occurs, as observed by a splitting of the lowest fundamental frequency into two frequencies close together :fl, ,( 13) = 0.7685 min-l andf,, ,( 17) = 0.9087 min-l.One can see even a subharmonic, if,. There is a very strong periodicity of period length two at T = 496 K, as seen from the graph of the attractor. In the linear plot of the Fourier spectrum the period which is expected because of the pitchfork bifurcation according to the Feigenbaum scenario, cannot be detected.28 For this purpose one needs to look at the logarithmic power spectrum (see fig. 7). Besides the three fundamentals fl( 14), f2(29) and f3(44) one can detect the subharmonics of fl, :fl and ifl, and the ultrasubharmonics ifi. The ultrasubharmonic :fl closely resembles the subharmonic $f2. Furthermore, two linear combinations are located close tof2(29), 2f1 = f(28) and 2f2 = JT30), which are locked by $,(29).Looking at the higher harmonics off, in the logarithmic power spectrum one can estimate f, more exactly to be fl( 14, 2). The third frequency, f3(48), and its harmonics can be neglected with respect to the outstanding structure of the time series, which is obviously controlled by f, and f, and their sub- and ultrasub-harmonics. At 492 K two strong frequencies, f1(15) and f2(33), and their linear combination dominate the Fourier spectrum. Upon lowering the temperature this behaviour is continued at 498 K and then develops into a spectrum of only one main frequencyf'(24) and its subharmonics and harmonics at 486 K, while the pattern of the time series begins to show a fractal structure with small repeating areas (see fig.5). The existence of subharmonic frequencies if,, ifl and the ultrasubharmonic frequency ifl as well as the harmonic frequency 2f1 can easily be seen in fig. 6 at 486 K. The fractal structure developed at 476 K can mainly be described by the two frequencies of the doubletf,, ,(22), fi, ,(24), their subharmonics and linear combinations (see fig. 7). The higher frequency, &(56), loses its influence41 the structure of the spectrum and the time series. The shape of the spectrum becomes much simpler again for some special low temperatures, showing only one frequency and its subharmonics and ultrasubharmonics. Fig. 8 shows two examples for highly developed fractal patterns at low temperatures ( T = 468 K and T = 461 K), which differ greatly in the complexity of their spectra. To obtain better resolution we have taken 300 harmonics for the Fourier analysis in these cases.The number of the frequency has now to be multiplied by the factor y1z = 1.47 x lo-, to obtain the frequency in cycles min-'. As was the case for 476 K, the subharmonics of the old fundamentals now become the main frequencies of these spectra. For instance, the 468 K spectrum is determined by three new frequencies g0(26), g,(49) and g,( 59), which derive from the frequencies g( 1 1) = ifp, ,( 1 l), f,, ,(22) and f,, ,(24) in the spectrum at 476 K. In the linear spectrum at 468 K one can recognize the subharmonics f g0(9), f go( 13), 5 g,( 16), + g2( 19), the ultrasubharmonics f go( 17), 8 g,(32), f g,(40) and several linear combinations : (g2 -gl) ( lo), (go + g, - 8,) (36), (go + 8,) (7% (g2 - 5 8,) (43).Again the higher frequencies1762 L-4 x c w- 1" Oscillating Catalytic Oxidation of CO I ir 7 IN L O lamod h 0 v 1: m %.- =-I? 0 i, iamod A aJ Y m w iamod h Q v - x Y N x ? 4 ? m 1 i m cv x 0, 4 9 : i m -0 samod &iamod0 0.5 1.1 1.6 2 . 2 2 . 7 3.3 3 . 8 4 . 4 cycles min-' fl 2 j fl I 0 0.5 1.1 1 g = 2f1,l f l , l ( h ) P 4 i 1.6 2.2 2 . 7 3.3 3.8 4*4 cycles min-'1764 Oscillating Catalytic Oxidation of CO h 0 v 0 N II N, c \I, * m II N (Y x 00 * II N m I . k N N N c x m N Y, h( -. Y,- * N *-. m b, CDP. J. Plath, K. Moller and N . I. Jaeger 1765 can be understood by frequency locking, e.g. (2 g, - 2 g, +go + g,) (65) = f g,(65) ; Lowering the temperature the new frequencies, g, increase and lose their influence on the spectrum, whereas their own subharmonics h and linear combinations of them now become important.At 461 K the spectrum becomes very simple again, because of a strong frequency locking between the subharmonics and their differences. On the one hand it seems as if only one frequency i(7) and its harmonics will rule the whole spectrum, but on the other hand one can derive this spectrum from that at 468 K: the former frequencies g0(26), g1(49) and g,(59) are shifted and split now, yielding g0,,(36), go, ,(43), gl, ,(56), gl, ,(64), g2, ,(77) and g2, ,(86). 'I'heir diff~~ences ( g2,, -gl, (2% (g,, -gl, 1) (2 l), (gl, , -go, ,) (21) are very close to the subharmonic h2(22) = + go, ,. The difference (go, , -go, ,) (7) just equals i(7) the subharmonic h,(22) and + hl( M), whereas h,( 14) = 5 go, ,(43).h,(29) = f go, , can also be detected easily. Furthermore, g0,,(43) can be understood as a subharmonic of g,,,(86). It seems that the temperature dependence of the time series can be expressed by the following scenario : first, the system can be characterized by three fundamental frequencies, the ratio of which resembles a harmonic one. Lowering the temperature increases the amplitudes of these frequencies (see fig. 9), while subharmonics and ultrasubharmonics become more and more important and locking of frequencies occurs. The higher frequencies then lose their influence on the spectrum. Decreasing the temperature causes bifurcation mainly of the lowest fundamental frequency. The spectra become very simple, essentially structured by only one frequency and its subharmonics and ultrasubharmonics, if the fractal character of the time series is fully developed [see fig.5(h) and fig. 6(h)]. Highly chaotic patterns arise [see fig. 6 0 and (i)] if additional bifurcations occur,31 creating a tremendous number of new linear combinations. The chaos in our system seems to be deterministic and temperature is an order parameter. There are small areas of this parameter Ton which strong periodic behaviour occurs, which can essentially be described by one fundamental frequency and its subharmonics or harmonics. There are two tendencies which lead to chaos: (a) lowering the temperature the fundamental frequencies increase, giving rise to bifurcation and (b) subharmonics and ultrasubharmonics occur on this line, taking over the leading role in the spectra by frequency locking.The spectra become simple even for highly developed fractal patterns of the time series if all main frequencies, the fundamental as well as their subharmonics and ultrasubharmonics, are well locked. In this case all frequencies occurring in the spectra seem to be harmonics of the lowest frequency, the amplitude of which becomes the highest. The time series correlated to such spectra show a clear fractal character, which can be modelled assuming an excitation wave travelling across a fractal framework to a Sierpinsky gasket.15 (2g,+;gz-g1)(88) zs gg,(88,5); ~g,(73)+(g,+g,)(75)(148) 3 g1. The Model To model this behaviour by using a one-dimensional cellular automaton as described e a ~ l i e r ~ ~ ' ~ ~ a re-injection procedure has to be established in order to obtain a periodic pattern similar to the experimental results.This can be achieved by giving the automaton a finite length. The automaton consists of a finite number of cells arranged linearly. The state of each of these cells can be one or zero, corresponding to the producing and non- producing state of the catalytic units or the Pd and PdO phases, respectively. The whole catalyst is represented by such a column of cells. The amplitude of the break-downs of the CO, production, which is assumed to be proportional to the amount of catalytic units in the PdO state, is now correlated to the number of cells of the zero-state. The expansion of the non-active PdO phase is simulated by a propagation of state-changes of the cells (1 -+ 0) in one direction only, starting from a cell whose state is always zero.If1766 Oscillating Catalytic Oxidation of CO A 0 Y JaMOd2 56 3 8 4 t l s 0 12 8 1 1 I I I I I I I t 1 I I I I I ( b ) I hl h2 h3 c 0) a II. d. 111 I,.I,II. 1.1 . I . , , I I I L . l * . ~ , 1 I I I . 1 1 2.70 4.05 1.35 $ cycles min-' 0 P Ir B cu op w 2 Fig. 8. Time series and logarithmic plots of the Fourier analysis (300 harmonics): (a) T = 468 K, go = 26, g, = 49, g 2 = 59; = 43, gl,l = 56, g1,2 = 64, g2,I = 77, g2,2 = 86, h, = 14, h2 = 22, h, = 29, i = 7. (b) T = 461 K, go,1 = 36, g0.21768 Oscillating Catalytic Oxidation of CO 50 40 rz 01 VI 8 2 30 - I c 2 2 20 m c( h 10 0 A \ 2 00 210 220 2 30 T/"C Fig.9. Fundamental frequencies of the power spectra taken as a function of the reactor temperature T. Fig. 10. Time development of one-dimensional automatons. Fundamental repetition units characterized by different AF. (a) Y not defined, u = 8, A F = 0.25, K(u) = 2; (6) r not defined, u = 4 , A F = 0 . 5 , K ( u ) = ~ ; (c) r = 2 , u = 2 , A F = 1 . 0 , K ( u ) = ~ ; ( d ) r = 3 , u = 2 , A F = 1 . 5 , K(u) = 3 ; (e) r = 4, u = 2, AF = 2.0, K(u) = 4.P. J. Pluth, K. Mdler and N . I. Jaeger 1769 Fig. 11. Variation of the length 1 of the one-dimensional automaton creating smallest new sub- harmonics $ of the fundamental frequency F,: K(u) = 2, u = 2 and F, = + in all cases. (a) E = 8, p = 8, Fp = i; (b) E = 10, p = 16, Fp = &; (c) I = 12, p = 16, Fp = &; ( d ) E = 24, p = 32, Fp = &.one cell that is non-catalytic (blocked) touches this boundary of the limited automaton at time n, the time development of the automaton will be stopped. The remaining K(n) blocked cells would produce K(n + 1) blocked cells at time (n+ 1) without reaching a boundary. However, beyond a boundary line no more blocked cells can be produced. The automaton is now re-injected into the starting position, but the point of departure only contains B(n + 1) < K(n + 1) blocked cells because of the boundary, which cuts off the rest K(n + 1) - B(n + 1). The ,automaton now develops starting with the B(n + 1) blocked cells remaining in the position in which they have been produced at time n. In this manner the automaton becomes periodic at time p , whereas the unrestricted automaton can never achieve periodicity apart from its dilatational symmetry.At time p the number K(p) of blocked cells is equal to the length 1 of the restricted automaton. It has to be mentioned that p and not n, the point of re-injection, determines the period length. As we have seen from the Fourier analysis of the experiments, not only one frequency governs the system, but in most cases two or three frequencies and their linear1770 Oscillating Catalytic Oxidation of CO combinations. The ratio of such basic frequencies is nearly an integer. Furthermore, the experiments have shown that the frequencies depend on the temperature of the reactor: the higher the temperature the lower the frequencies. With respect to our model, frequency has to be defined by the number K(u) of blocked cells at the end of the smallest repetition unit.u is the number of time steps, for which all blocked cells are adjacent, forming a blocked row of cells of length K(u) # 1. The structure of this fundamental repetition unit is just the structure, which creates the dilatational symmetry of the automaton. Now a frequency can be defined as F = l / u correlated with the amplitude A = K(u). Fig. 10 shows some examples for different frequencies F and different products AF. In the experiments AF and Fare small for high temperatures. This can be achieved in the model either by using a low value for A = K(u) and a high value for u or by using very small values for r, where r(r 2 u) is the number of cells at time (n+ 1) following an isolated cell at time n [ref.(1 5)]. For very high temperatures no cells can be blocked, even though a blocked cell might exist at the beginning (stationary point), whether F or AF go to zero. The limit for low temperatures implies the existence of a very high frequency, the upper limit for which is F = i. The simulation of the experiment by the proposed model results in a diminished possibility for the formation of blocked cells with increasing temperature. By lowering the temperature more catalytic units can be blocked within a repetition unit, but beside the fundamental frequency, subharmonics can be observed in the experiments. The amplitudes correlated to these subharmonics determine the structure of the time series increasingly with lower temperature.This means that lowering the temperature increases not only r but also the length 1 of the automaton, whereas F tends to its upper limit. These subharmonics, 4, of the fundamental frequency, F,, = l/uo, can be defined by the time ui (i = 1, 2, ...) for which all blocked cells of the automaton are adjacent, forming a row of length K(u,): 4 = l/ui. The smallest subharmonic is determined by Fp = l/up = l/p, where up = p is the repetition time of the automaton. Increasing 1 with decreasing temperature the smallest new subharmonics are born at distinct values of I (see fig. 11). Now, if r becomes a discrete function of I, as for example in ref. (1 5), different K(u) and therefore different fundamental frequencies, F, will occur, However, because of the structure of the automaton the development of the subharmonics of all fundamental frequencies are of great importance with respect to the structure of the time series.This corresponds to the fact that the subharmonics of all fundamental frequencies in the experimental time series are of great importance for their global structure. Indeed, these subharmonics enable the determination of the rules governing the cellular automaton. Conclusion Fourier analysis of the time series leads to a deeper understanding of the dynamics of CO oxidation catalysed by Pd crystals supported on an amorphous A1,0, carrier. The resulting spectra are essentially structured by frequency locking. The temperature dependence of the temporal structure of the process can be modelled qualitatively by a one-dimensional automaton, as introduced in ref.(15) and (16). To understand why it is possible to describe the complex dynamics of this catalytic system simply in this manner one has to reflect on the fact that the cooperative effect is arranged by an excitation wave travelling along the two-dimensional network of Pd particles in the catalyst supported on the silver sieve or ceramic support. Assuming that this network is a fractal one because of the amorphous character of the A1,0, carrier and the random distribution of the pellets in the catalyst bed, the number of Pd particles touched by a cyclic wave at time n should be correlated to the state of the one-dimensional automaton at a corresponding time interval n.P. J. Plath, K. Moller and N .I. Jaeger 1771 Although the experimental facts can be described qualitatively by this model, this should not be overemphasized. For instance, there is a stochastic nature within the experimental results, which the complex but nevertheless strong periodicity of this model does not reflect. Furthermore, in an excitable two-dimensional Euclidean space spiral waves should be observable. To model such space-time by an automaton it has to be two-dimensional at least. The authors are grateful for financial support from the Stiftung Volkswagenwerk. References 1 N. I. Jaeger, K. Moller and P. J. Plath, J. Chem. Soc., Faraday Trans. I , 1986, 82, 3315. 2 C . Wagner, Ber. Bunsenges. Phys. Chem., 1970, 74, 401. 3 W. Keil and E. Wicke, Ber. Bunsenges. Phys. Chem., 1980, 84, 377.4 G. Ertl, P. R. Norton and J. Rustig, Phys. Rev. Lett., 1982, 49, 177. 5 M. P. Cox, G. Ertl, R. Imbihl and J. Rustig, Surf. Sci., 1983, 134, L 517. 6 D. Bocker and E. Wicke, in Temporal Order, Springer Series in Synergetics, Vol. 29., ed. L. Rensing and 7 M. P. Cox, G. Ertl and R. Imbihl, Phys. Rev. Lett., 1985, 54, 1725. 8 A. Dress, N. I. Jaeger and P. J. Plath, Theor. Chim. Ada, 1982, 61, 437. 9 N. I. Jaeger, P. J. Plath and E. van Raaij, Z. Naturforsch., Teil A , 1981, 36, 395. N. Jaeger (Springer-Verlag, Berlin, 1985), pp. 75-85. 10 A. Th. Haberditzl, N. I. Jaeger and P. J. Plath, Z. Phys. Chem. (Leipzig), 1984, 265, 449. 1 1 M. Gerhardt, H. Schuster and P. J. Plath, Ber. Bunsenges. Phys. Chem., 1986, 90, 1040. 12 H. Beusch, P. Fieguth and E. Wicke, Chem. Ing. Tech., 1972, 44, 445. 13 N. I. Jaeger, K. Moller and P. J. Plath, 2. Naturforsch., Teil A , 1981, 36, 1012. 14 N. I. Jaeger, K. Moller and P. J. Plath, in ref. (6), pp. 96-100. 15 N. I. Jaeger, K. Moller and P. J. Plath, Ber. Bunsenges. Phys. Chem., 1985, 89, 633. 16 A. W. M. Dress, M. Gerhardt, N. I. Jaeger, P. J. Plath and H. Schuster, in ref. (6), pp. 67-74. 17 J. E. Turner, B. C. Sales and M. B. Maple, Surf. Sci., 1981, 109, 591. 18 T. Engel and G. Ertl, A h . Catal., 1979, 28, 2. 19 D. D. Eley and P. B. Moore, Surf. Sci., 1981, 111, 325. 20 P. W. Davies and R. M. Lambert, Surf. Sci., 1981, 111, L 671. 21 S. Ladas, H. Poppa and M. Boudart, Surf. Sci., 1981, 112, 151. 22 G. Vayenas and N. Mickels, Surf. Sci., 1982, 120, L 405. 23 D. Ruelle, Math. Zntell., 1980, 2, 126. 24 E. Ott, Rev. Mod. Phys., 1981, 53, 655. 25 R. H. G. Hellemann, in Fundamental Problems in Statistical Mechanics ed. E. G. D. Cohen (North Holland, Amsterdam, 1980), vol. 5, pp. 165-233. 26 R. Shaw, in Chaos and Order in Nature, ed. H. Haken, Springer Series in Synergetics, Vol. 1 1 (Springer-Verlag, Berlin, 198 l), pp. 218-231. 27 0. E. Rossler, Z. Naturforsch., Teil A,, 1976, 31, 259. 28 J. P. Eckmann, Rev. Mod. Phys., 1981, 53, 643. 29 H. G. Schuster, Deterministic Chaos (Physik-Verlag, Weinheim, 1984), pp. 91-1 36. 30 C. Vidal, Springer Series in Synergetics Vol. I I (Springer-Verlag, Berlin,. 1981), pp. 69-82. 31 S. Fauve and A. Libchaber, in ref. (30), pp. 25-35. Paper 612109; Received 30th October, 1986

ISSN:0300-9599    

DOI:10.1039/F19888401751

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(6), 1773-1778 L-Edge EXAFS Studies of the Coordination of Lead in PbO-PbF, Glasses B. Govinda Rao and Kalya J. Rao Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India Joe Wong General Electric Corporate Research and Deuelopment, Schenectady, New York, 12301, U.S.A. Room-temperature EXAFS above the Pb L, absorption edge in a series of PbO-PbF, glasses containing 8WO mol % PbO have been measured with synchrotron X-rays from CHESS at Cornell. The amplitude and phase parameters have been obtained from the parent crystalline compounds PbO and PbF, for the analysis of the EXAFS of glasses. The EXAFS results confirm that Pb is indeed present in [PbO,F,]-type units in all compositions, as predicted by the structural model of these glasses proposed earlier based on X-ray diffraction studies.Glasses are formed in wide ranges of composition in the systems PbO-PbF, and PbO-PbC1,.1.2 We have investigated the structure of these glasses by a variety of techniques. A structural model was developed for PbO-PbCl, glasses based on X-ray diffraction ~tudies.~ A significant feature of the structure of PbO-PbC1, glasses is that lead is present in an octahedron of the type [PbO,Cl,] in glasses of all compositions. The structural model accommodates this feature by allowing the halogen coordination to vary. The coordination of lead was also established by EXAFS4 and neutron diffraction5 studies. Further, we have found that PbO-PbFq glasses are extremely similar to PbO-PbC1, glasses.In particular, the pair distribution functions obtained from X-ray diffraction studies are strikingly similar in various features for glasses of both systems. Therefore, it has been proposed elsewhere' that the principal feature of the structure of PbO-PbF, glasses is also the presence of octahedrally coordinated Pb in [Pb0,F4] units. A direct confirmation of local coordination in glasses is possible by employing extended X-ray absorption fine structure (EXAFS) In the case of PbO-PbC1, glasses4 the EXAFS associated with the L, edge of Pb were analysed using the phase parameters obtained from a similar analysis of Pb &-edge EXAFS of the parent compounds, namely PbO and PbC1,. A multi-shell simulation procedure was employed to fit the experimental data.In the present study, however, a slightly modified procedure is adopted, and it has been shown that Pb is indeed present in [PbO,F,]-type units in all PbO-PbF, glasses. Experimental PbO-PbF, glasses with 80,60,50 and 40 mol % PbO were prepared for this investigation. Weighed quantities of yellow Pb,04 and PbF, (all AR grade) in required proportions were mixed thoroughly and melted in short quartz tubes using an oxyacetylene flame until a uniformly clear melt was obtained. The melt was kept at this temperature (ca. 900-1000 K) for a short time (2-3 min), poured onto a polished steel plate and pressed quickly with another polished steel block. The glass discs thus obtained were transparent 17731774 EXAFS Studies of Pb in PbGPbF, Glasses 3 3.00 2.60 0 4.00 3.2 0 2 40 - 4 r -400 400 1200 0 2.80 -400 4 00 1200 3.60 1 r u 2.8.400 400 I200 E lev Fig. 1.Room-temperature L, absorption spectra of Pb in PbO, PbF, and PbO-PbF, glasses. (a) PbO, (b) PbF,, (c) 80 Yo PbO, ( d ) 60 % PbO, (e) 50 YO PbO and (f) 40 YO PbO. - 0.01 o-ol u - O.O33,O0 7.00 11 .OO n A! x v 0.01 -0.01 (c) 0.0 4 c o.ooi -0.04- 3.00 7.00 1l-00 0.01 - 0.01 - - 0.02 3.00 7.00 11.00 4 .OO 8.00 12.00 4 .OO -0.03 1,1111 -0.03 k1A-I Fig. 2. Normalized EXAFS, ~ ( k ) us. k for PbO, PbF, and PbO-PbF, glasses. (a) PbO, (b) PbF,, (c) 80 % PbO, ( d ) 60 O/O PbO, (e) 50 740 PbO and cf) 40 YO PbO.B. G. Rao, K. J. Rao and J. Wong 1775 and essentially colourless. These were powdered to < 400 mesh and mixed with Duco cement. The slurry was cast into films by spreading it between two microscopic slides which were then slid apart to expose two identical films, and those were allowed to set in air.The concentration of the glass powder in the slurry and the film thickness were manipulated so that in the resulting film (either single or duplex) the concentration of lead across the thickness of the film is equivalent to 1-2 absorption lengths at the L, absorption edge of Pb (13035.0 eV). The EXAFS above the PbL, edge of the glasses was recorded with the C-2 spectrometer at CHESS (the Cornell High Energy Synchrotron Source) facility at Cornell with CESR (the Cornell Electron Storage Ring) operating at an electron energy of ca. 5.3 eV and an injection current of ca. 12-15 mA. The absorption spectra were recorded at room temperature, and no precautions were taken to avoid moisture, since the glasses are not sensitive to humidity. Samples of PbO and PbF, were also prepared in the same manner and the spectra were recorded under identical conditions.Results and Discussion The room-temperature Pb L,-edge absorption spectra of four different compositions of PbO-PbF, glasses along with those of PbO and PbF, are presented in fig. 1. Data reduction followed a standard procedure'? of de-glitching, pre-edge and post-edge background removal, edge normalization, extraction of the EXAFS signal ~ ( k ) , Fourier transformation of x(k) and inverse transformation to isolate the EXAFS contribution from a selected region in real space. The pre-edge background was obtained by a linear regression analysis of the first ten raw data points. The post-edge background in the EXAFS region was generated analytically using a series of cubic SplineslO of equal segments.The energy scale wa! converted into the k scale using the relationship k = [2m(E-Eb)]5/A = [0.263(E-Eb)]F where E,, is the edge energy of the Pb L, edge.ll The normalized EXAFS, ~ ( k ) , at energies above 30 eV was obtained by subtracting the smooth post-edge background, po(k) from the measured absorption, p(k) and dividing by the step jump, S at the absorption edge and with a McMaster', correction, m(k), as a function of energy The normalized EXAFS of PbO and PbF, along with those of the glasses are shown in fig. 2. The noise level in the EXAFS of the compound, PbO and 80 PbO - 20 PbF, glass, is quite high.The data were Fourier-transformed and the structure functions, cD(r), so obtained are shown in fig. 3. Because of the high level of noise in the EXAFS of 80 mol % PbO glass, the corresponding structure function contained many spurious Qeaks, and hence the analysis of this glass is not reported here. The radial peak at ca. 2.0 A was inverse Fourier-transformed in each case to obLain the contribution from the first coordination sphere of lead in the region 0.7-2.7 A. The inverse transforms are plotted in fig. 4 as solid lines. In the single scattering approximation the observed ~ ( k ) is given by6.' (2) 1 ~ ( k ) = - - 2 Aj sin [2rj k + k i where the EXAFS oscillation frequency arising from each shell, j , is given by the sine term and its amplitude is given by Aj.A, is also k-dependent and is given by N . r; A,(k) = --lf.(n, k ) exp (- 2rj/A) exp (- 2 4 k2). (3)1776 0.08- 0.04 EXAFS Studies of Pb in PbO-PbF, Glasses - h W e N 2 o.04k 0.00 0.00 4.00 8.00 0.16 0 . 0 8 L 0.00 0.00 4.00 8.00 ( d ) 0.08 0.00 1- 0.00 L O O 890 0.08 r-l o.04ki 0.00 0'00 4.00 8.00 rlA Fig. 3. Fourier transform plotted as @(r) us. r of the corresponding EXAFS of fig. 2. (a) PbO, (6) PbF,, (c) 60% PbO, ( d ) 50% PbO and (e) 40% PbO. n 5 0.04 r- v - 0.04 1 l i l t 2.00 6.00 10.00 a x . I 0.01 I- A 0.00 - 0.01 _ _ 2.00 6.00 10.00 0.01 - 0.01 - 0.03 4.00 8.00 12.00 k1B-l 0.01 - 0.01 u -omoik 8.00 12.00 Fig. 4. Inverse Fourier transform (solid line) and simulated EXAFS of the first peak in @(r). (a) PbO, (b) PbF,, (c) 60 % PbO, ( d ) 50 % PbO and (e) 40 YO PbO.B.G. Rao, K. J . Rao and J . Wong Table 1. Structural parameters of PbO-PbF, glasses compo- coordinating sition atom in the standard PbO:PbF, shell Ni r , / A deviation 60 : 40 0 2 2.23 1.18 50: 50 0 2 2.21 1.32 40 : 60 0 2 2.20 1.23 F 4 2.59 F 4 2.57 F 4 2.53 1777 Eqn (2) and (3) are characterized by (a) the scattering parameters q5j(k) (phases), f,(n, k ) (amplitudes) and A (mean free path of the scattered electrons) and (b) the structural parameters Nj (the number of scatterers), rj (the distance of the scatterers) and aj (the disorder parameters containing both a Debye-Waller-type disorder and the spread in the static scatterer distances). In these parameters j represents the shell giving rise to the isolated EXAFS contribution.The scattering parameters for the analysis of the EXAFS of glasses were obtained by fitting the inverse transforms of the model compounds with the calculated EXAFS. In the present case, PbO and PbF, are the model compounds. Since Nj and rj are known for these compounds, the quantity J(n, k) exp ( - 2rj/A) exp ( - 24k2) can be determined directly from the maxima and minima of the EXAFS oscillation^.^^ From the values of k corresponding to the maxima, minima and nodes in the inverse transforms of the model compounds, the phase shift $,(k) for the various atom pairs can be determined.13 The expression for the phase shift q5j(k) has the following form: q5W = Po 4- PI k 4- P2 k2 4- P3/k3. (4) The above method has been employed successfully in the analysis of EXAFS of catalyst systems.I3 The phase parameters so obtained for Pb-0 and Pb-F pairs are given below: q5Pb--0 = - 13.29- 1.12k-0.0049k2- 12.03/k3 q5Pb--F = - 15.17- 1.19k-0.0083k2+62.09/k3.These phase parameters, along with the amplitude functions from the tabulated values of Teo and Leo, gave a good fit to the experimental inverse transforms of PbO and PbF,. The simulated spectra are plotted as crosses in fig. 4(a) and (b). These parameters are very similar to the reported Pb-0 phases (obtained4 by multi-parameter curve fitting) and Pb-F phases.14 In particular, the second term, which is k-dependent, agrees quite well in both the cases. Therefore, these phase parameters were employed in the analysis of the EXAFS of PbO-PbF, glasses. The curve-fitting of inverse EXAFS data to eqn (2) was performed with two subshells (one Pb-0 subshell and one Pb-F subshell) to obtain coordination around the lead in glasses.Since the results of X-ray diffraction and other studies suggest that the coordination polyhedron around lead is of the type [Pb02F4] for all glass compositions, we tried initial fits with an oxygen subshell of two oxygens and a fluorine subshell of four fluorines while fitting the inverse transforms. Reasonable fixed values of Q were used for the purpose. The generated XF(k) is shown by crosses in fig. 4. The fits are quite satisfactory. The optimized values of r and standard deviation of fitting are given in table 1. Table 1 shows that the Pb-0 and Pb-F distances systematically decrease with1778 EXAFS Studies of Pb in PbO-PbF, Glasses PbF, concentration, which indicates that the [PbO,F,] units become more closely packed in PbF,-rich glasses.Such an observation is also consistent with the evolution of symmetry of [PbO,F,] units as suggested in the XANES studies of these g1a~ses.l~ The [PbO,F,] units become more symmetric and also more ionic at higher PbF, concentrations, and as a result they become more closely packed. Concluding Remarks Only average values of Pb-0 and Pb-F distances in PbO-PbF, glasses are obtained by the method of analysis employed in the present work. A multi-shell analysis such as in the case of PbO-PbCl, glasses would have given more accurate values of individual Pb-0 and Pb-F distances. However, the main objective of this work was to show that [PbO,F,] units are present in all compositions of PbO-PbF, glasses.The present method of analysis proves to be quite adequate in fulfilling this objective. References 1 B. G. Rao, H. G. K. Sundar and K. J. Rao, J. Chem. SOC., Faraday Trans. 1, 1984, 80, 3491. 2 K. J. Rao, B. G. Rao and S. R. Elliott, J. Mater. Sci., 1985, 20, 1678. 3 B. G. Rao and K. J. Rao, Phys. Chem. Glasses, 1984, 25, 1 1. 4 K. J. Rao, J. Wong and B. G. Rao, Phys. Chem. Glasses, 1984, 25, 57. 5 A. C. Wright, D. I. Grimley, R. N. Sinclair, K. J. Rao and B. G. Rao, J. Phys. (Paris), 1985, C8, 305. 6 J. Wong, in Topics in Applied Physics, ed. H. J. Guntherodt and H. Beck (Springer-Verlag, Berlin, 1980), vol. 40, chap. IV. 7 K. J. Rao and B. G. Rao, Bull. Mater. Sci., 1983, 7, 353. 8 F. W. Lytle, D. E. Sayers and E. A. Stern, Phys. Rev. B, 1975,11,4825; E. A. Stern, D. E. Sayers and F. W. Lytle, Phys. Rev. B, 1975, 11, 4836. 9 P. A. Lee, P. H. Citrin, P. Eisenberger and B. M. Kincaid, Rev. Mod. Phys., 1981, 53, 769. 10 C. DeBoor, J. Approx. Theor., 1968, 1, 219. 11 J. A. Bearden and A. F. Burr, Rev. Mod. Phys., 1967, 39, 125. 12 W. H. McMaster, N. Nerr del Grande, J. H. Mallet and J. H. Hubbell, Compilation of X-Ray Cross- sections (Lawrence Radiation Laboratory, UCRL 50/74 1969), sect. 2. 13 G. Sankar, S. Vasudevan and C. N. R. Rao, J. Chem. Phys., 1986,85,2291; Chem. Phys. Lett., 1986, 127, 620. 14 N. Kamizo, K. Koto, Y. Ito, H. Maeda, K. Tanabe, M. Haida and H. Terauchi, J. Phys. SOC. Jpn, 1984, 53, 4210. 15 K. J. Rao, B. G. Rao and J. Wong, J. Chem. SOC., Faraday Trans. 1 , 1988, 84, 1779. Paper 6/23 19 ; Received 1st December, 1986

ISSN:0300-9599    

DOI:10.1039/F19888401773

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(6), 1779-1794 Investigation of the Coordination of Lead in PbO-PbF, Glasses using XANES Kalya J. Rao and B. Govinda Rao Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Joe Wong General Electric Corporate Research and Development, Schenectady, New York 12301, U.S.A. The Ledge spectra of lead in PbO-PbF, glasses have been investigated. Since PbO-PbF, glasses contain [PbO,F,] units throughout the glass- forming compositions and since PbF,-rich glasss are considerably more ionic than PbO-rich glasses, the symmetry of the [PbO,F,] units increases from C, through C,, to D,, and possibly to 0,. The changes in symmetry have been ascertained from XANES using appropriate molecular-orbital diagrams.We have shown that the splitting of the L, and L, edges, the variation of the half-widths of all three edges and the Ledge intensities are indeed consistent with symmetry changes in the [PbO,F,] units and the suggested molecular-orbital diagrams. While the extent of ionicity of bonding undergoes a continuous increase as the PbF, content in the glass is increased, the white-line intensities at the edges decrease in the same manner. This has been shown to be a consequence of the deshielding of final states responsible for the Ledge spectra by evaluating the dipole-matrix elements using Slater orbitals. The evolution of the symmetry of the [PbO,F,] units, the variation of Ledge absorption intensities and the variation of ionicity have thus been shown to be directly related.X-Ray absorption near-edge structure (XANES) arises from the excitation of electrons from core levels such as ls(K), 2s(L1), 2p,,,(L,), 2p3,,(L3) etc. to the lowest allowed unoccupied electronic states in the atom.lv2 The high-energy side of the absorption edge consists of an undulatory continuum of absorption which exponentially decreases in intensity. Absorption undulations above 30-50 eV from the edge constitute the extended X-ray absorption fine structure (EXAFS), which contains direct structural information. 3-5 Both EXAFS and XANES are finding increasing applications in investigations of structure and bonding in both crystalline and amorphous The L-absorption edge of an element is often very sharp and well defined, and is referred to as a white line.1*2.7 White lines appear when the final state is a discrete allowed atomic level or a band state with a high amplitude of an orbital of allowed symmetry on the absorber atom.'-1° Since the Ledge transitions are associated with p and d final states,l' they reflect the extent to which p and d final states are affected by the nature of bonding1, and by the symmetry of the coordination polyhedronf3 around the atom.Hence the analysis of the XANES can give rise to vital chemical information, namely local structure and bonding. Glasses can be formed in the PbO-PbF, system over a wide range of c~mpositions.~~ We have ~ h o w n l ~ * ~ ~ that the PbO-PbF, glass structure is built up of [PbO,F,]-type octahedral units throughout the glass-forming compositions. The nature of bonding in the components of this glass system, namely PbO and PbF,, is widely different16 (highly covalent in PbO and highly ionic in PbF,).Hence the ionicity of Pb-0 and Pb-F 5 9 59 1779 F A R 11780 XANES Spectra of Pb in PbO-PbF, Glasses bonds in [PbO,F,] units can be expected to exhibit a continuous variation as a function of composition. Since increased ionicity decreases the rigidity and directionality of bonds we may expect the more ionic PbF,-rich region to consist of more symmetrical [PbO,F,] units. Such a change of symmetry and bonding may be expected to affect the Ledge spectra which involve 1s -+ 6 p and 2p -+ 6d transitions of lead as a function of composition; hence the relevance of XANES of Pb in these investigations. Bonding in [PbO,F,] units requires hybridization of the 6s, 6p and 6d states on lead in order to accommodate in them a significant degree of covalency of bonding.In fact such hybridization of valence orbitals has been invoked in the bonding models of pure Pb0.l' An important consequence of hybridization in XANES studies is the incidence of new features in Ledge spectra arising from otherwise forbidden transitions. This aspect of bonding is therefore particularly well suited to examination using XANES.1S-20 The Ledge absorption intensity itself can be used to infer the nature of bonding because electrons involved in covalent bonding screen the final states relevant to Ledge transition~.~l-~~ Hence variation of Ledge absorption intensities in the binary series of PbO-PbF, glasses can reflect the trend in the nature of bonding.explain the near-edge feature of X-ray absorption spectra. Further, no band-structure calculations are available for any of these glasses. Hence an MO approach has been assumed to be quite adequate for our discussions. We may, however, note that these approaches do not adequately take care of the effect of core hole^.^^.^^ Also, lead atoms are separated by intervening oxygen and fluoride atoms, which limit the overlap of p and d orbitals of Pb while forming bands. An atomic description of the spectroscopic phenomena may not therefore be unrealistic. With this background we have examined the consequences upon the XANES spectra of the evolution of symmetry in [PbO,F,] units as a function of PbF, concentration, because with increasing PbF, content the glasses become more ionic and the [PbO,F,] units tend to become more symmetrical. In our analysis we have considered excitations up to 30 eV above the edge as the upper limit of XANES, though recent reports suggest even higher energy limits.,, Other features such as the L,/L, intensity ratio and variations of half-widths and energies of the various peaks in the XANES spectra are also informative about the structure of these glasses, and we have discussed these aspects using the same MO description.Symmetry-based molecular-orbital (MO) theory has been widely invoked''. 24-33 to Experimental The PbO-PbF, .glasses used in this study were prepared by melting together appropriately weighed quantities of Pb,O, (AR) and PbF, (AR) in quartz tubes and quenching drops of melt between polished steel plates (Pb,O, decomposes quantitatively to PbO at ca.840 K). The details of the method of preparation have been reported ea~1ier.l~ Samples for recording XANES were prepared by mulling fine powders (400 mesh) ground from transparent thin discs of the glass with Duco cement and casting the mull into thin films between microscope slides. Details of these casting procedures have been given el~ewhere.,~ The concentrations of glass powders in the mull and the thickness of the films were manipulated so as to provide two absorption lengths of the material at energies just above the L,, L, and L, edges of lead with the use of one or two such films. Only pinhole-free films (by visual examination over a light box) were used.Uniformity of thickness was better than 5 O h over a 2 in length,? and no radiation damage was detected after beam exposure. Room-temperature spectra were obtained with the C-2 spectrometer at CHESS (the Cornell High Energy Synchrotron Source) with CESR (the Cornell Electron Storage Ring) running at 5.3 GeV and an injection current of 12 mA. The synchrotron X-ray t 1 in = 2.54 x lo-' m.K. J. Rao, B. G. Rao and J . Wong 3.60 s: p 2.80 E - 2.00 E .r. Y 0.80 8 2 a, s 3 0.40 I I 1 I I I 15800 16200 16600 17000 1: 60 - 20 20 60 EIeV EIeV 1781 I Fig. 1. (a) Room-temperature Pb L, absorption spectrum of lead foil, schematically showing the edge normalization procedure (see the text). (b) Normalized edge spectrum in the range f 60 eV. The zero of energy is taken with respect to the L, edge of Pb at 15860.8 eV.beam from CESR was monochromatized with a channel-cut Si(220) crystal and a 1 mm entrance slit. The incident beam was detuned 50 % to minimize harmonic contents at the Pb L,, L, and L, energies.,* This yielded a resolution of 5 eV at the lead L, edge at 13035.0 eV, which is comparable to the estimated life time broadening of the Pb L, edge of 4.6 eV. Spectra were recorded in three energy regions about the respective L, (13035.0 eV), L, (15200.0 eV) and L, (15860.8 eV) edges: -200 to -50 eV in 10 eV steps; - 50 to 50 eV in 1 eV steps; and above 50 eV in 3 eV steps. This scanning procedure yielded quality data of both pre-edge and post-edge backgrounds for subsequent normalization of the XANES spectra.The spectrometer was calibrated with a Pb foil before, between and after scanning the lead compounds and glasses. No spectrometer drifts were detected over a period of two days of continuous scanning. The procedure for obtaining normalized XANES spectra is similar to that used in an earlier study of lead compounds and PbO-PbC1, gla~ses.~’ This is briefly described here for the L, spectra of Pb metal. Fig. 1 (a) is a plot of In (Z,,/Z) us. energy around the Pb L , edge energy at 15 860.8 eV. The pre-edge region XY (- 200 to - 50 eV) is linearly fitted and extrapolated above the edge(E = 0) to Z. The post-edge (EXAFS) background AB to 500 eV above the edge was determined by using a cubic spline fit procedure with equal segments (usually three) and extrapolating below the edge to C.The absorption jump is given by PQ at E = 0. The near-edge spectrum is then normalized by subtracting the line XZ from all points in the region & 60 eV and dividing the difference by the edge jump PQ to yield the normalized spectrum shown in fig. 1 (b). In the case of L,, L, and L,. edges the zeros of energy were taken with respect to the corresponding threshold of photoelectron ejection at 13 035.0, 15 200.0 and 15 860.8 eV, re~pectively.,~ These threshold energies were calibrated with the first peak in the corresponding derivative spectra. The derivative spectra in the range -20 to 40 eV were obtained by drawing through points given by dA/dE = [A(E+A)-A(E)]/A (1) where A is the energy step size in the absorption spectra. Thus various inflection points in the normalized XANES spectra show up as peaks in the derivative spectra, and the absorption maxima of the XANES spectra correspond to zero on the decreasing parts of the derivative spectra.The half-widths at half-maximum (h.w.h.m.) of the main 59-21782 XANES Spectra of Pb in PbO-PbF, Glasses absorption peaks in normalized XANES spectra were determined as the difference between the energies of the peak and the zero on the high-energy side of the peak in the derivative spectrum (extrapolated wherever necessary). Areas under the absorption peaks were determined by deconvoluting the XANES peaks into Gaussians by a non- linear fitting program. In the case of the L, edge the spectra were fitted to one Gaussian, while two Gaussians were necessary for the L, and L, edges.The pre-edge absorptions were ignored in these fits (see later). Reasons for, and limitations of, using Gaussian fit instead of Lorentzian fit are discussed later. Results and Discussion Results Normalized XANES spectra of crystalline PbO, PbF, and four PbO-PbF, glasses containing different percentages of PbO are presented in fig. 2 for the L, (a), L, (b) and L, (c) edges. The differentiated spectra are presented in fig. 3(a), (b) and (c), in the same order. The h.w.h.m. is indicated for one spectrum in each case, for illustration. As pointed out earlier, the peaks corresponding to allowed transitions 2s + 6p (for the L, edge) and 2p -+ 6d (for the L, and L, edges) have been identified with reference to the L edges of a Pb foil. The energies of other absorptions in the near-edge spectra are also listed in table 1 for all three edges and are again referred uniformly to the appropriate L edges of a Pb foil. Variations of the energies of the pre-edge and post-edge peaks associated with the L, and L, edge are shown in fig.4. The L, edge does not appear to be associated with the subsidiary peaks. The variation of the h.w.h.m. of the principal absorptions is shown in fig. 5, and the variation of the area under the principal peak, as a function of composition (for all the three edges), is presented in fig. 6. General Features of XANES Pb is formally in its divalent state in PbO and in all the glasses. The 6p and 6d states of Pb are therefore completely vacant, and one should expect reasonably intense white lines for the three Ledge transitions since they involve vacant final states.The L, transition in PbF, is associated with typically well-defined white-line characteristics. Contrary to their appearance on cursory examination, the differentiated spectra of fig. 3 suggest that the h.w.h.m. of PbO and the PbO-PbF, glasses are also similar to the h.w.h.m. of the &-edge absorption of PbF,. Values of the h.w.h.m. of the various L-edge absorptions are listed in table 1 along with the peak energies. Since the absorption peaks at the edges are sufficiently well defined it appears that the transitions occur to discrete final states. From table 1 and fig. 2 and 3 we note that there are two additional transitions, which may be described as pre-edge and post-edge absorptions, respectively, associated with L, and L, spectra, while L, spectra have no such features.These additional features can arise from crystal-field splitting of the 6d levels on Pb in a low-symmetry coordination polyhedron. However, such low symmetries would also split the 6 p levels, whereas the &-edge spectra, being free of shoulders, do not support this possibility; hence, considering lead to the present as simple Pb2+ ions in a crystal field is not adequate. We may note in this context that Pb-0 bonding is quite covalent in pure PbO, and also the Pb-F bond can be quite covalent in the glassy state, as suggested in several investigations of PbF,-based glasses. l4> 37,38 Hence, we consider that MO description of bonding in [Pb0,F4] octahedra is appropriate.The origin of the observed XANES spectra may then be discussed in the light of an MO model. In the crystal structure of yellow PbO it is found16 that Pb is coordinated to four oxygens at two unequal distances and that all oxygens are present on the same side ofK. J. Rao, B. G. Rao and J. Wong 1783 -60.00 -20.00 20.00 60.00 EIeV 1.20 0.80 0.40 0 -60.00 -20.00 20.00 60.00 I c E/eV 0 EIeV Fig. 2. Normalized spectra of the L, (A), L, (B) and L, (C) edges of Pb in PbO, PbO-PbF, glasses and PbF,. The ordinate scale corresponds to the PbO spectrum. Other spectra are systematically displaced for clarity. %PbO = (a) 0, (b) 40, (c) 50, ( d ) 60, (e) 80 and (f) 100. the lead atom. This gives rise to a pyramidal [PbO,] unit with C, symmetry. When PbF, is added in order to form glasses, Pb atoms acquire mixed oxygen-fluorine coordination.This results in a decrease of the oxygen coordination of an average Pb atom. The novel feature of these glasses, as revealed by X-ray diffraction is that in the entire glass-formation range lead is coordinated to two oxygens and four fluorines.0.12 0.08 0.04 0.00 -0.04 -24.00 -8.00 8.00 24.00 EIeV -0.04 40.00 l o \ 0.12 0.08 0.04 0.0c K C - EIeV Fig. 3. Derivative spectra of the L, (A), L, (B) and L, (C) edges of Pb in PbO, PbO-PbF, glasses and PbF,. The ordinate scale corresponds to the PbO spectrum. Other spectra are systematically displaced for clarity. The half-width at half maximum (h.w.h.m.) is indicated for PbO spectrum in each case. The energies (in eV) of the peak shoulders are also indicated.K .J. Rao, B. G. Rao and J. Wong 12.0 0 I I I I I 1 1785 Fig. 4. Variation of the energies of pre-edge and post-edge peaks associated with L, and L, edges with composition. These energies for PbO and PbF, are also marked in the figures. A, L, pre- edge; a, L, pre-edge; A, L, post-edge and 0, L, post-edge. l x b I I I I I 0 20 40 60 80 100 Pb5 PbFz (mol%) Fig. 5. Variation of the h.w.h.m. of the principal absorption of all the three edges with composition. The h.w.h.m. for PbO and PbF, is also marked in the figure. x , L, edge; A, L, edge and 0, L, edge. PbO Further, the analysis of the EXAFS associated with the Pb L, edge of these glasses shows that Pb atoms are in coordination polyhedra of the type [PbO,F,]. The results of the EXAFS analysis are reported e1~ewhere.l~ Such a structure helps retention of Pb-0-Pb linkages which, as we suggested earlier, is important for the stability of lead oxyhalide glasses.Further, the glass structure requires fluoride ions to possess variable coordination.1786 XANES Spectra of Pb in PbO-PbF, Glasses 9.5 n .; 9.0 d a g 8.5 a 3 v 8.0 X 0 20 40 60 80 100 PbO PbFz (mol%) Pb5 Fig. 6. Variation of the normalized area under the principal absorption of all the three edges with composition. These areas for PbO and PbF, are also marked in the figure. x , L, edge; A, L, edge and 0, L, edge. As the PbF, concentration increases we should expect an increase in ionicity of bonding, and the structure of [PbO,F,] units should undergo a continuous shape modification.The evolution of symmetry of [PbO,F,] units may be visualized as follows. Initially the two longer (weaker) Pb-0 bonds" in PbO are broken to accommodate the newly introduced Pb atoms from added PbF,. Fluoride ions from the added PbF, would now surround the Pb atoms generating the [PbO,F,] octahedra. Such octahedra in the PbF,-poor compositions can possess only C, symmetry because the initial 0-Pb-0 bond angle is close to a right angle. At high concentrations of PbF,, however, an average Pb-0 bond becomes more ionic, the 0-Pb-0 angle opens up into a large obtuse angle, the fluoride ions readjust their position and the symmetry around the lead atom increases to CZv. At much higher concentrations of PbF,, the ionicity of the Pb-0 bond becomes even higher, and the 0-Pb-0 units can become almost linear.This allows a rearrangement of the octahedra into considerably more symmetrical, more tightly packed [PbO,F,] units of D,, symmetry. Since oxygen and fluorine ions possess similar nephelauxetic character,39 and also since their sizes are nearly equal (0,- and F- ions may be treated as indistinguishable in the ionic limit), the [PbO,F,] unit may be considered as an octahedral unit of Oh symmetry. The evolution of the geometry of the [PbO,F,] units suggested in this work is indicated in fig. 7. The assertion that the geometry of high-PbF, glasses becomes highly packed is supported by our EXAFS investigations. l5 Molecular-orbital Approach and XANES Tentative MO diagrams relevant to bonding in [PbO,F,] units in different geometries are presented in fig.8. It has been assumed in drawing the MO schematics that only the 2p orbitals on the fluorine and oxygen atoms are important for ligand bonding and that they are of similar energies. The 6s, 6p and 6d orbitals of Pb are all assumed to be required in bonding. Note that the 6d orbitals of Pb must be utilized in order to account for bonding to six ligands and in order to conform to the symmetry requirements in the various geometries. Further, 6p and 6d levels of Pb atoms are ca. 8 and 1 1 eV above itsTable 1. Absorption peak energies a and their widths for the absorption features in the L,, L, and L, edges absorption absorption absorption compound or glass half-width/eV energy/eV half-width/eV energy /eV half-width/eV energy/eV PbO 5.56 0 6.0 -8.8, 0, 8.8 8.6 -7.1, 0.0, 11.5 80 PbO : 20 PbF, 6.18 0 7.7 -8.8, 0, 8.8 8.2 -7.7, -0.6, 10.3 60 PbO : 40 PbF, 6.14 0 6.4 -8.8, 0, 8.8 8.4 - 10.3, - 1.3, 9.0 50 PbO : 50 PbF, 6.00 0 6.5 -8.8, 0, 8.8 9.1 -8.8, - 1.9, 8.9 40 PbO : 60 PbF, 5.96 0 6.5 -8.8, 0, 8.8 8.3 - 10.3, - 1.9, 7.7 PbF, 5.66 0 6.4 0, 10.5 5.6 2.6, 10.9 a See experimental section, paragraph 3 for the calibration and scaling procedure used.Table 2. Symmetry species of ligand and metal orbitals and of Pb-0 bonds in the four point groups ligand orbitals metal orbitals 2 4 , +A,, +B1, + E u 6s A’(s) A&) A&) 6P A’&, P,) + A”@,) A,@,) + Bl@J + B,@J Eu@, +PJ + A,,@,) 6d A’(+) + A”(dzz) 4 ( d z z ) + B,(d,,) A,,(4+ + 4,(dz2-y2> two Pb-0 bonds A’ + A” A1 +Bl Al, + A,, (2p of 0 and F) 4A’ + 2A“ 3A1 + 2B1 + B, Table 3.The final states for the L,, L, and L, absorptions of Pb in the different point groups L2 L3 point pre-edge principal edge post-edge pre-edge principal edge post-edge group Ll 4a’* 2b: 1 e,* 5a’ (or 6a’, 3a’) 2a’* 4a‘* 3a” (or 6a’, 5a’) a”* 4a1 (or 1 a,, 2b,) 1 b: 26: 2b, (or la,, 4a1) 2a: 4 1 e* lb,,(or le,) a,*, 4 leg (or 1b2,) t 2 , n 00 t 2 , - W U -1788 XANES Spectra of Pb in PbO-PbF, Glasses (C) ( d ) Fig. 7. Geometry of six-coordinated Pb in PbO-PbF, glasses in various symmetries. The symmetry evolves as a function of composition (see text). (a) C,, (b) CZv, (c) D,, and (6) 0,. 0 , Pb; 0, R@' 0 and@, O/F. 6s levels, re~pectively,~' which is not prohibitive for hybridization. Since [PbO,F,]'- units carry 10 valence electrons they are accommodated in the first five molecular orbitals of lowest energy.In table 2 the symmetry species of the metal orbitals in the four point groups are indicated, along with the species designation of the Pb-0 bonds. The metal 6s orbital is assumed to be involved in the formation of the lowest-energy MO. In the C, point group, metal-oxygen bonds which form only a slightly obtuse 0-Pb-0 angle make use of the two p-orbitals hybridized with the 6s orbitals. In C,, symmetry the 0-Pb-0 angle becomes more obtuse and requires the use of hybridized (dxz-pz) orbitals for the formation of one Pb-0 bond and hybridized (s-p,) orbital for the other. In PbF,-rich compositions, where the symmetry of the [PbO,F,] unit becomes D,,, the 0-Pb-0 angle opens out to 180".The two Pb-0 bonds may now be formed using a hybridized (di-s) orbital and a p , orbital of the metal atom. Note also that the axis of symmetry is turned by 90" as we change from the C,, to D,, point group, which accounts for the sudden change of the p-orbital labelling in table 2. We have identified the fourth and fifth molecular orbitals in our bonding scheme as utilizing largely the d orbitals in C, and C,, symmetries. In D,, symmetry the fifth level is one of the degenerate e, orbitals which uses 6p orbitals of Pb. However, since the symmetries in glasses cannot be expected to be perfect, these levels may become non-degenerate, allowing one of the levels to become totally unoccupied, as indicated in fig. 8. In all three symmetries the lowest unoccupied MO is therefore formed from a metal 6p orbital and is energetically well separated from other higher vacant orbitals.The lowest unoccupied metal orbitals of d symmetry are non-bonding in character. In D,, symmetry they are made up of a doublet and a singlet level, whereas in C, and C,, symmetry the non-bonding levels are made up of singlets. It is reasonable to assume further that these non-bonding levels are affected by the ligand field in a higher order of perturbation and to different extents by the coordinating ions. In these three symmetries the non-bonding d levels are separated from the bonding orbitals on the low-energy side of antibonding p orbitals which have the same effective symmetry as d orbitals. Similar antibonding p orbitals are present on the higher-energy side of non-bonding d orbitals in these symmetries.In 0, symmetry the splitting of the levels is quite simple and is generated by a 'bunching up' of several levels of the D,, MO diagram of fig. 8.K. J . Rao, B. G. Rao and J . Wong 1789 6d 3 6P 6s la1 I a** A ---l&J a' Pb 0, F4 Pb 02 5 Fig. 8. Molecular-orbital diagrams of ebO,F,] units in various symmetries: (a) C,, (b) C,,,, (c) D,, and ( d ) 0,. (See text for definition of the symbols.) It is evident that the lowest unoccupied MO level not only has metal p-character, but also is present in the lower three symmetries and is moreover separated from filled orbitals on the low-energy side and from the unfilled orbitals on the higher-energy side. This feature of MO diagrams accounts for the well defined single-peak character of the L, absorption edge of fig.2. The L2,3 edges are due to the transitions to empty orbitals which are of uniformly non-bonding character. Below these non-bonding d orbitals empty antibonding pa orbitals are present in these cases. Since transitions to antibonding pa levels are not forbidden, prominent lower-energy shoulders appear with L2,-edge absorptions. The absorption peak on the higher-energy side of the L2,3 spectra may possibly arise from transitions to antibonding p-type orbitals on the higher-energy side of non-bonding d orbitals. These levels are indicated in table 3 for these symmetries. Thus all the principal features of the L-edge spectra are qualitatively consistent with the MO description. Note also that in the MO diagrams of fig.9 the retention of two Pb-0 bonds and the opening up of 0-Pb-0 bond angle from near 90" in the pyramidal [PbO,] units present in parent PbO to 180" in [PbO,F,] units of D,, symmetry in PbF,-rich glasses appears quite consistent and natural.1790 XANES Spectra of Pb in PbO-PbF, Glasses g Fig. 9. Variation of calculated adsorption coefficient, p, as a function of g , the number of electrons transferred from the metal p orbital to the ligand orbital (in the MO picture shown in fig. 8). The dashed line corresponds to the L, edge and the solid line to the L2,3 edges. The L, absorption is due to the transition 2p,/, --* 6d,/,, while the L, absorption arises from the transition 2p,,, + 6d,,,. Since any of these transitions results in less than half- filled shells, the higher-energy orbitals may be assumed to constitute the d5,, (higher-j] statesg and hence the levels indicated in table 3.We further assume that the non-bonding higher-j orbitals are spatially so directed that they are always more perturbed (the perturbation is, however, only of a higher order). In comparison, the d3/, (lower-I] states are less perturbed by the symmetry changes of the octahedron. We should like to stress that the magnitude of these perturbations is low, and indeed for the L, edge it is insignificant; however, the L, edge is stabilized (see table 1). The energy of the post-edge absorption peak associated with the L, edge decreases as a function of PbF, concentration (fig. 4). This is equivalent to a decrease in the energy gap as a function of the evolution of the symmetry of [PbO,F,] units.Note from fig. 8 that as the symmetry changes from C, to C,, to D,, the energy of one of the p a orbitals increases, crosses that of the da orbitals and rises to the top of the bonding orbitals. The energy of the corresponding antibonding p a orbital is lowered. Since the particular antibonding p a orbital has been identified as the final state responsible for the post-edge absorption, a lowering of its energy is reflected in fig. 4. Since the other p a orbital retains its relative energy with respect to the other bonding orbitals (always being associated with the lower-energy p , orbital in table 2), the corresponding antibonding orbital is essentially unaffected in energy. Since this is the antibonding level responsible for the post-edge peak associated with the L, edge, its energy remains unaffected (fig.4). The pre-peaks also behave roughly similarly. Their origin is attributed to the excitations to antibonding levels formed from metal p orbitals. Corresponding bonding orbitals are unoccupied, as pointed out earlier. While the L, pre-edge behaviour is understandable in the sense that these final states are essentially unaffected, the behaviour of L, pre-edge is difficult to rationalize. At this point we feel that this may be a specific final-state effect which is more stable for higher effective-. states (for electrons excited from 2p,,, levels). The variation of the h.w.h.m. as a function of composition is shown in fig. 5 , which is again consistent with the MO diagrams of fig.8. The L, edge arises from a transitionK. J. Rao, B. G. Rao and J. Wong 1791 to a single discrete p level in all glasses, and the h.w.h.m. values are therefore generally low. L, and L, edges are associated with considerably high values of the h.w.h.m., since the final states possess a spread of energy. When the symmetry evolves to there is a noticeable decrease in the h.w.h.m. values of the L, edge absorption, and this is consistent with the occurrence of a lower-energy (lower--) non-bonding doublet of d levels. The h.w.h.m. values of the L, edges are the largest. Perhaps this is due to the larger spatial extension of the corresponding higher-j orbitals which overlap with similar orbitals of neighbouring [PbO,F,] units and give rise to band-type character.Absorption Intensities and Covalency The absorption intensities have been calculated from areas under the L-edge absorption peaks treating the peaks as Gaussians rather than as Lorentzians. The error introduced by this procedure is likely to be both systematic and low. Since we intend to evaluate only the systematic changes as a function of composition, this procedure is adequate. The variation of the areas is shown in fig. 6 as a function of PbF, content in the glass. The areas decrease systematically as the PbF, content increases, or equivalently as the ionicity of the glass increases, because an average Pb-0 bond can be expected to become more ionic as PbF, content increases. This general feature of a larger white-line intensity in a more covalently bonded situation (such as in a glass) has been noted by us in the studies of Ledges of Nd3+ and Th4+ in the glasses and crystals.21y22 It is gratifying to note that in the present instance a correlation of white-line intensities with ionicity is evidenced in the glassy phase itself, in which the bond ionicity has been varied by varying the composition.Referring to the MO diagrams of fig. 8, an increase in the ionicity amounts to a flow of electrons from the bonding orbitals to the ligands, creating vacancies or decreased density of electrons in the participating metal orbitals. Removal of electrons from the s or p orbitals of Pb unshields the metal p and d orbitals which are the final states for the L-edge transitions. The reduction in shielding affects the final state orbitals, thereby affecting the transition-dipole matrix elements, ( ~ y , ~ 1 ra 1 v 6 d ) and ( vZs I ra I f y 6 p ) ' In general the magnitudes of the matrix elements decrease with a reduction in shielding (see later), and hence the absorption intensity, which is determined by the square of the matrix element, decreases.Therefore, as the PbF, content increases in the glass, the Ledge absorption intensities decrease. We wish to make a semi-quantitative evaluation of this effect by calculating the matrix elements using Slater functions for the I,Y,~, yZp, ~y~~ and ysd orbitals. Assuming that in a perfectly covalent situation, 10 electrons of the first five occupied bonding orbitals (of fig. 8) are equally shared between the metal atom and the ligands, Pb atoms may be formally associated with five electrons; two 6s and three 6p electrons. We can set out to evaluate the absorption coefficient for this situation using the one-electron approxi- mation and with Koopman's theorem.The absorption coefficient, p, is given by4' p = (47~/n) (e2/hc) holMij12 6(Ei - Ej -hm) (2) where Mij = (il ra l j ) is the dipole matrix element connecting the two states (il and lj), S is the Kronecker delta, Ei and Ej are the initial and final state energies, n is the refractive index, o is the frequency of absorption and c is the velocity of light. The states (il and ( j ) correspond to the 2s and 6p levels, respectively, in the case of the L, edge, and to the 2p and 6d levels, respectively, in the case of the L,,, edge of Pb.With Slater functions the j states are not distinguished, and the functions themselves are nodeless. Mij for the L, and L2,, edges are given by1792 Slater functions have the form4, XANES Spectra of Pb in PbO-PbF, Glasses Un,, s Un = rn*-l exp [-(Z-S)r/n*] ( 5 ) where Z is the atomic number and S is the screening constant, which is calculated using Slater's rules.42 For the above (fully covalent) situation, the values of S relevant to 2s/2p, 6p and 6d are 4.15, 77.05 and 79.75, respectively, and the n* values corresponding to the principal quantum numbers 2 and 6 are 2.0 and 4.2, respectively. As the ionicity increases we assume that g electrons are transferred to the ligands. When g = 3 it would leave Pb in the divalent Pb2+ state, and this situation is assumed to be completely ionic. Since the electrons are transferred from the p levels of Pb, the screening constants relevant to the 6p and 6d wavefunctions decrease.The reduction of the screening constant is found to be 0.35g and l.Og, respectively, for the 6p and 6d wavefunctions. Using relatively simple algebra one can show that the transition-dipole matrix elements are given by M i = y(6.2)/(40.472 + 0. log)", (6) and M i = y(6.2)/(39.622 + 0.3 1 3g)6.2 (7) pL1 = (KmLla2)/(bl + c1g)12.4 (8) pL,., = (Ko,2,,a2)/(b2 + c2d12.4 (9) where y(x) are the gamma functions. Therefore p L l and p L , , , can be written as where K , a, b,, b,, c, and c, are constants ( K = 4ne2/nc, a = r(6.2) = 169.41, b, = 40.472, c1 = 0.109, b, = 39.622 and c, = 0.313).p L , and pL, have been evaluated for various values of g and are shown in fig. 9 as a function of g. Note that the nature of the variation of p as a function of g is similar to that of the absorption intensity as a function of the PbF, content in the glass. Thus an increase in the bond ionicity of g causes a decrease of the white-line intensity. It is difficult to make an absolute comparison between the absorption coefficients obtained for various degrees of electron transfer (fig. 9) with absorption intensities shown in fig. 6 obtained as a function of the PbF, content. However, it is instructive to compare the fractional changes occurring in the absorption coefficients of fig. 9 with the fractional variation of absorption intensities of fig. 6. In fig. 9 p changes by ca.10 and 30% for the L, and L, edges, respectively, when bonding becomes completely ionic. In fig. 6 there is a variation of ca. 3.2 and 6.4%, respectively, in the intensities for the L, and L, peaks when the PbF, concentration is changed from 20 to 60 mol% in the the glassy range. If we further make the approximation that the relative changes in the total absorption intensities at the L, and L, edges are directly related to relative changes in p of fig. 9, then the change in the ionicity of the glasses is in the range of 22-32 O/O. Such a change appears to be plausible. The semiquantitative approach made here is admittedly inadequate for at least two important reasons. The first is that Slater functions are nodeless and make no distinction between j states, since they are independent of both 1 and s.Secondly, the 2s and 2p electrons of Pb should be treated in a relativistic framew~rk,~, and no account has been made of this effect. However, it provides a basically sound qualitative explanation of the trend in the variation of absorption intensities as a function of ionicity. Also, the relative magnitudes of the L, and L, absorption intensities from these calculations are quite satisfactory (cf. fig. 6 and 9). L,/L, Absorption Intensity Ratios Another aspect of the XANES spectra which has been discussed in the literature at considerable length is related to the ratio of the L, to L, absorption intensities. We haveK. J . Rao, B. G. Rao and J . Wong 1793 Table 4. Experimental amplitude ratio, L,/L,, and calculated free volume per Pb atom, 5 experimental calculated" amplitude free volume ratio, per Pb atom, glass composition L J L , Y 80 PbO : 20 PbF, 1.304 29.06 60 PbO : 40 PbF, 1.467 30.19 50 PbO : 50 PbF, 1.429 30.48 40 PbO : 60 PbF, 1.492 30.69 a = (M/p-4/3, Nx,r:) 1024/N, where A4 is the experimental molar volume, xi and ri are the mole fraction and the radius of the ion, respectively, and N is the Avogadro number.[See ref. (44) for details.] computed this simply from the ratios of the absorption peak heights in the raw spectra for the various glass samples, and give these data in table 4. These ratios are typically < 1.5 and are higher for more ionic samples. Since they appear to be affected by the ionicity of bonding in the glass, which is a chemical effect, it suggests that the j character at the non-bonded d orbitals varies as a function of ionicity.The larger j states may be considered as having a slightly higher spatial extension (j being considered in place of E in the corresponding spherical harmonics). Conversely, slightly higher volumes are preferred by the high-j non-bonding d orbitals of Pb. In fact the available free volume44 per Pb atom increases with increasing PbF, in the glass (see table 4), which suggests that the 6d states of Pb are perhaps characterized to a greater extent by j = states in PbF,-rich compositions. This accounts for the general increase of the L3/L2 absorption intensity ratio as a function of increasing ionicity . Nevertheless, the ratio is itself considerably lower than the theoretical value of 2.0.It is therefore possible that the initial 2p,,, and 2p,,, populations are effected by the relativistic behaviour of the electrons in the Pb atoms. Conclusions XANES has been employed to study the changes in the nature of bonding in PbO-PbF, glasses. Since the two important features of these glasses are that (a) Pb is present in coordination polyhedra of the type [PbO,F,] in the entire glass-forming range and (b) the nature of bonding changes as a function of composition; the symmetry of [PbO,F,] units changes with the composition, or equivalenty as a function of ionicity from C, to CZv to D,, symmetry. This results from a change from directionally rigid covalent Pb-0 bonds to relatively loose, non-directional ionic bonds, as discussed using plausible MO diagrams.The variation of XANES features, such as pre-edge and post-edge positions etc. is consistent with such an evolution in geometry. The intensities of the L, and absorption peaks (given by areas under the peaks) also vary in a manner consistent with the increasing ionicity of PbF,-rich glasses. The absorption coefficients have been calculated from transition4ipole matrix elements in the one- electron approximation and with Slater orbitals, and have been used to justify the L,3/L2 intensity ratios in the experimental Ledge absorptions. Additionally, these calculations yield an estimate of ca. 22-32 % change in ionicity in the composition range 2MO% PbF,.1794 XANES Spectra of Pb in PbO-PbF, Glasses We are grateful for the experimental opportunity at CHESS which is supported by N.S.F., U.S.A.K.J.R. and B.G.R. thank Prof. C . N. R. Rao for this kind encouragement and D.S.T. India for financial support. Our grateful thanks are also due to a referee for helpful suggestions for improving the manuscript. References. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 I L. V. Azaroff and D. M. Pease, in X-Ray Spectroscopy, ed. L. V. Azaroff (McGraw-Hill, New York, 1977), chap. 6. Synchrotron Radiation Research, ed. H. Winick and S. Doniach (Plenum Press, London, 1980). P. A. Lee, P. Citrin, P. Eisenberger and B. Kincaid, Rev. Mod. Phys., 1981, 53, 769. J. Wong, in Topics in Applied Physics, ed. H. J. Guntherodt and H. Beck (Springer, Berlin, 1981), vol.46, chap. 4. EXAFS Spectroscopy Techniques and Applications, ed. B. K. Teo and D. C. Joy (Plenum Press, New York, 1981). EXAFS and Near Edge Structure, ed. A. Bianconi, L. Incoccia and S. Stipcich (Springer-Verlag, New York, 1983). Y. Cauchois and N. F. Mott, Philos. Mag., 1949,40, 1260. S . W. Gupta and B. D. Padalia, J. Phys. F, 1971, 1, L16. M. Brown, R. E. Peierls and E. A. Stern, Phys. Rev. B, 1971, 15, 738. P. S. P. Wei and F. W. Lytle, Phys. Rev. B, 1979, 19, 679. Y. Cauchois and C. Bounelle, C.R. Acad. Sci. Paris, 1975, 245, 1230. J. A. Bearden and T. M. Snyder, Phys. Rev., 1941, 59, 162. U. C. Srivastava and H. L. Nigam, Coord. Chem. Rev., 1972,9, 275. B. G. Rao, H. G. K. Sundar and K. J. Rao, J. Chem. SOC., Faraday Trans. I , 1984, 80, 3491.B. G. Rao, K. J. Rao and J. Wong, J. Chem. SOC., Faraday Trans. I , 1988, 84, 1773. A. F. Wells, Structural Inorganic Chemistry , (Oxford University Press, Oxford, 1975). B. Dickens, J. Inorg. Nucl. Chem., 1965, 27, 1495, 1503. D. J. Nagel and W. L. Baum, in X-Ray Spectroscopy, ed. L. V. Azaroff (McGraw-Hill, New York, 1977), chap. 9. K. J. Rao and J. Wong, J. Chem. Phys., 1984,81,4832. J. Wong, F. W. Lytle, R. P. Messmer and D. H. Maylotte, Phys. Rev. B., 1984, 30, 5596. K. J. Rao, J. Wong and M. J. Weber, J. Chem. Phys., 1983, 78, 6228. K. J. Rao, J. Wong and M. G. Shafer, J. Solid State Chem., 1984, 55, 110. G. C. Long, A. G. Revesz and M. Kuriyamma, J. Non-Cryst. Solidr, 1985, 70, 271. P. E. Best, J. Chem. Phys., 1966, 44, 3248, G. L. Glenn and C. G. Dodd, J. Appl. Phys., 1968, 39, 5372. W. Seka and H. P. Hanson, J. Chem. Phys., 1969, 50, 344. D. W. Fischer, J. Appl. Phys., 1970, 40, 3561. R. G. Shulman, Y. Yafet, P. Eisenberger and W. E. Blumberg, Proc. Natl Acad. Sci. USA, 1976, 73, 1384. K. Ichikawa, 0. Aita, H. Nakamori, M. Kamada and K. Tsutsumi, Jpn J. Appl. Phys., 1978,17, suppl. K. Tsutsumi, 0. Aita and K. Ichikawa, Phys. Rev. B, 1977, 15, 4638. M. Belli, A. Scafali, A. Bianconi, S. Mobilio, L. Palladino, A. Reale and E. Benattini, Solid State Commun., 1980, 35, 355. L. A. Grunes, R. D. Leapman, C. N. Wilkes, R. Hoffman and A. B. Kunz, Phys. Rev. B, 1982, 25, 7157. L. A. Grunes, Phys. Rev. B, 1983, 27, 2111. T. K. Sham, Phys. Rev. B, 1985, 31, 888. K. J. Rao, J. Wong and B. G. Rao, Phys. Chem. Glasses, 1984,25,57; J. Wong, Nucl. Instrum. Method, in press. J. A. Berarden and A. F. Burr, Rev. Mod. Phys., 1967, 39, 125. S. Shibata, T. Kanamori, S. Mitachi and T. Manabe, Mater. Res. Bull., 1980, 15, 129. J. P. Miranday, C. Jacoboni and R. De Pape, J. Non-Cryst. Solidr, 1981, 43, 393. D. Ahrland, J. Chatt and N. R. Davies, Quart. Rev., 1958, 12, 265. C. E. Moore, Atomic Energy Levels, Natl Bur. Stand. Circ. No. 467 (US. GPO, Washington, D.C., F. C. Brown, in Synchrotron Radiation Research, ed. H. Winck and S. Doniach (Plenum Press, New York, 1980). J. C. Slater, Phys. Rev., 1930, 36, 57. L. M. Matheiss and R. E. Dietz, Phys. Rev. B, 1980, 22, 1663. K. J. Rao, G. Parthasarathy, B. G. Rao and E. S. R. Gopal, Mater. Res. Bull., 1984, 19, 1221. 17-2, 157. 1958), V O ~ . 111, pp. 207-218. Paper 7/546; Received 25th March, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401779

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1988, 84(6), 1795-1805 The Dimer State of NO in Micropores of Cu(OH),-dispersed Activated Carbon Fibres Katsumi Kaneko," Ai Kobayashi, Takaomi Suzuki, Sumio Ozeki and Kazunori Kakei Department of Chemistry, Faculty of Science, Chiba University, Yayoi, Chiba 260, Japan Nobuhiro Kosugi and Haruo Kuroda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan The magnetic susceptibility x of Cu(OH),-dispersed activated carbon fibres [Cu(OH),-ACF] with and without adsorbed NO was measured in the temperature range 298-423 K in order to clarify the micropore filling of supercritical gaseous NO. Cu(OH),-ACF was characterized by N, adsorption and X-ray absorption spectroscopy (EXAFS and XANES). The micropores of Cu(OH),-ACF are very uniform and have larger pore volumes than those of non-modified ACF.EXAFS and XANES suggested ultrafine Cu(OH), particles highly dispersed on the ACF. The x value of Cu(OH),-ACF with 125-135mgg-' NO was negative at 298K and increased with temperature, while the x value of Cu(OH),-ACF without NO was negative and independent of temperature ; the negative x is due to dimerization of NO in the micropores, and the increase of x with temperature arises from the dissociation of the NO dimer and desorption of NO. The dissociation enthalpy of the dimer in the micropores is larger than that of the condensed phase at low temperature by 6-15 kJ mo1-'. The relationship between NO dimerization and the micropore filling of supercritical NO was discussed.The adsorption of vapours in micropores, micropore filling, is enhanced by overlap of the potential fields from opposite walls. lP3 Micropore filling is different from capillary condensation, which can be described by the Kelvin equation. In microporous systems the Dubinin-Radushkevich (DR) equation has been used to describe micropore filling. We are interested in the micropore filling of supercritical NO. The three-dimensional critical temperature is 180 K and the two dimensional temperature can be estimated to be 90 K after de Boer.' As micropore filling is not effective for the supercritical gas, there was no good adsorbent for NO near room temperature. We prepared iron oxide-dispersed activated carbon fibres (Fe-ACF) which adsorb large amounts of NO up to 320 mg g-l adsorbent at 303 K by chemisorption-assisted micropore filling ;'-lo ultrafine iron oxides on the ACF assist the micropore filling of the supercritical NO. According to previous work," dispersion of CuO and the oxides and hydroxides of Ni and Co on the ACF brings about no chemisorption-assisted NO micropore filling ; however, dispersions of Cu(OH), and iron oxides do bring about such micropore filling.An NO molecule has an unpaired electron and is paramagnetic.12 Liquid and solid NO exhibit diamagnetism at low temperatures because of the disappearance of the unpaired electron due to NO dimerization. 13* l4 Recent magnetic susceptibility measurement of the non-modified ACF with adsorbed NO led the conclusion that NO Many researchers have paid attention to the mechanism of micropore 17951796 NO Dimers in Cu(OH),-ACF Micropores molecules adsorbed in the micropores are dimerized even above room temperature ;15 this phenomenon was caused by an adsorbate-adsorbate interaction enhanced by the micropore fields.However, we did not examine the dimerization of NO in the micropores of Fe-ACF owing to the large magnetic susceptibility of dispersed iron oxides. Cu(OH),-ACF can adsorb more NO than non-modified ACF and is diamagnetic. Dimerization of NO on Cu(OH),-ACF is expected to be observed more readily. Elaborate measurements of N, adsorption in the low-pressure range and examination of the compound dispersed on the ACF are indispensable in understanding the micropore filling of NO. In this work we measured the magnetic susceptibility of Cu(OH),-ACF characterized by N, adsorption isotherms with a special computer- controlled apparatus and by X-ray absorption spectroscopy (EXAFS and XANES). Experiment a1 Characterization We used pitch (PIT)- and cellulose (CEL)-based ACF.The method of sufface modification of ACF with Cu(OH), has been reported previously." Cu(OH), powder was obtained by adding 1 mol dm-3 NaOH to a mixture of 0.1 mol dm-3 CuSO, and 3 YO NH, solution at 343 K. Decomposition of the Cu(OH), powder at 423 K gave CuO. The adsorption isotherms of N, on the ACF samples at 77 K were measured using an automatic apparatus developed in our laboratory over a relative pressure range of 0.00005 to 1 .O (80 measurements) with a sensitivity of 5 Pa and 0.1 mg in order to obtain detailed information on the micropore structures.The samples were pre-evacuated at 383 K and lo-, Pa for 2 h before N, adsorption. Cu(OH),-ACF and non-treated ACF were examined by scanning electron microscopy (JEOL JSM-820). We measured the Cu K-edge XANES and EXAFS spectra of Cu(OH),, CuO and Cu(OH),-ACF using the EXAFS facilities at BL7C of the Photon Factory in the National Laboratory for High Energy Physics (Tsukuba, Japan). The Cu contents in Cu(OH),-ACF determined by the neocuproine method are 1 .O 0.1 wt O h for Cu(OH),-CEL and 1.2 0.5 wt YO for Cu(OH),-PIT. NO Adsorption The adsorption isotherms of NO at 303 K were measured gravimetrically by use of a quartz spring with a sensitivity of 5 mg mm-' and a cathetometer for gas pressures up to 80 kPa.The amount of adsorption was measured at 2 h after further introduction of gases (after 5 h from the first dose of NO). The preheating conditions for NO adsorption are similar to those for N, adsorption. The extent of irreversible NO adsorption was determined from the amount of residual gas after evacuation at 303 K and lo-, Pa for 2 h. NO (Takachiho Kagaku, 99.0% purity) was used after removal of NO, by vacuum distillation with dry ice-ethanol and liquid-N, traps. Magnetic Susceptibility Measurements The magnetic susceptibility x was measured by the Faraday method in the temperature range 298-423 K.15 80-100 mg Cu(OH),-ACF was packed in a 1 cm3 glass ampoule and then evacuated at 383 K and lop2 Pa; the ampoule was sealed after adsorption of NO on the ACF at 80 kPa and 303 K.The temperature dependence of x of synthesized Cu(OH), and CuO was measured in a similar way. The diamagnetic contribution of the ampoule was subtracted before the calculation of the magnetic susceptibility of Cu(OH),-ACF with and without adsorbed NO.J. Chern. SOC., Furuday Trans. 1, Vol. 84, part 6 Plate 1 Plate 1. Scanning electron micrographs: ( a ) CEL, (b) PIT, (c) Cu(OH),-CEL, ( d ) Cu(OH),- PIT. K. Kaneko et al. (Fming p . 1797)K . Kaneko et al. 1797 F 0 0.2 0.4 0.6 0.8 1.0 0 0.02 0.04 0.06 0.08 0.1 PIP, PIP, Fig. 1. Adsorption isotherms of N, on Cu(OH),-modified CEL (a) and modified CEL (0). 2oo t I I I I I I 1 I I I I 0 0.2 0.4 0.6 0.8 1.0 0 0.02 0.04 0.06 0.08 0.1 PIP, PIP, Fig. 2. Adsorption isotherms of N, on Cu(OH),-PIT (a) and unmodified PIT (b).Results and Discussion Micropore Structure CEL has folds along the fibre axis from SEM observation, as shown in plate 1, while PIT has no such folds. This difference is caused by the fact that source material of CEL origiflally has a textured structure, but PIT is composed of fibrous aggregates of ultrafine carbon particles.16 Cu(OH),-ACF has some deposits on the external surface of the fibre. We can see no micropores directly in these micrographs. Fig. 1 and 2 show the N, adsorption isotherms of Cu(OH),-ACF and untreated ACF. All isotherms are of type I, suggesting the presence of micro pore^.^' The surface modification with Cu(OH), increases the amount of N, adsorption, i.e. the micropore1798 NO Dimers in Cu(OH),-ACF Micropores 400 M I 1 I I I I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 t/nm Fig.3. t-plots for Cu(OH),-modified and unmodified ACF: 0, PIT; 0, Cu(OH),-PIT; 0, CEL; m, Cu(OH),-CEL. volume. Fig. 1 (B) and 2(B) show detailed adsorption isotherms at low relative pressure. There is a step near the relative pressure 0.02; the magnitude and positions of steps are characteristic of the sample. The step should be related to the two-step mechanism of micropore filling; we assume that N, molecules are adsorbed as a monolayer on both sides of the slit-type pore walls and then fill the N,-coated micropores. We will discuss this further in another report. We analysed t-plots for N, adsorption isotherms, in the construction of which the standard thickness values of N, adsorbed on carbon black'' were used.All t-plots bend near 0.4-0.5 nm to become nearly parallel to the abscissa. Fig. 3 shows the t-plots for Cu(OH),-ACF and untreated ACF. The specific surface areas, a,, from the t-plots are listed in table 1. a, values for the surface-modified ACF are greater than those of untreated ACF. We determined the micropore size distribution by the MP method; the micropores are very uniform and their widths are 0.8-1 .O nm. The micropore size of the ACF does not change during the Cu(OH), modification; the N, adsorption technique cannot distinguish reliably any difference in the micropore size as the micropores are ca. three times larger than the nitrogen molecule itself. The micropore-filling process can be roughly described by Dubinin-Radushkevich (DR) equation : where W and Wo are the amount of adsorption and the micropore volume, W = Wo exp ( - E,/E;) (1) E = RT In (Po/P) is the adsorption potential, Po is the saturated vapour pressure and Eo is the characteristic adsorption energy.Fig. 4 shows DR plots for the adsorption of N, on Cu(OH),-ACF. As our N, adsorption measurements gave unusual deviations from theK. Kaneko et al. 1799 Table 1. Adsorption parameters from t and DR plots CEL 1310 12 0.50 5.5 PIT 1230 5 0.47 5.9 Cu(0H) ,-PIT 1260 3 0.49 5.8 Cu(OH),<EL 1500 12 0.64 4.7 a Sext, external surface area. 0 10 20 30 40 50 InZ (POP) Fig. 4. DR plots for N, adsorption on Cu(OH),-modified and unmodified ACF. Symbols as for fig. 3. DR equation, we estimated Wo(N2) and Eo(N,) from the plots in the usual relative pressure range. The upward deviation around ln2(Po/P) = 30 for Cu(OH),-ACF is clearer than that for unmodified ACF.The dispersion of Cu(OH), seems to give rise to energetic heterogeneity in the micropores. The deviation from linearity will be discussed in another report. The adsorption parameters from t- and DR-plots are listed in table 1. The State of Cu Dispersed in ACF Since carbons are almost transparent to X-rays, we used X-ray absorption spectroscopy in order to characterize Cu(OH), on the ACF. X-Ray absorption near-edge structures (XANES) of the Cu K-edge of Cu(OH),-ACF, powdered Cu(OH), and CuO are shown in fig. 5. All four spectra are closely similar; each has a shoulder (B) about half-way up the Cu K-edge, which is characteristic of CuII c o r n p o ~ n d s .~ ~ ~ ~ ~ A weak pre-edge peak (A) arises from the 1s-3d transition which is forbidden in the case of the 0, symmetry around a Cu” ion. The intensity of peak A is so weak that 0, symmetry is almost1800 NO Dimers in Cu(OH),-ACF Micropores Fig. 5. XANES of Cu 8950 9000 9050 photon energy/eV K-edge of CuO (a), Cu(OH), (b), Cu(OH),-CEL (c) and Cu(OH),-PIT distance/nm Fig. 6. Fourier-transforms of the EXAFS oscillation, k3x(k), for CuO (a), Cu(OH), (b), Cu(OH),-CEL (c) and Cu(OH),-PIT (d).K. Kaneko et al. 1801 1501 I I 0 40 80 NO pressure/kPa Fig. 7. Adsorption isotherms of NO on Cu(OH),-modified ACF at 303 K. (-) Adsorption, (---) desorption. 0, 0, Cu(OH),<EL: ., 0, Cu(OH),-PIT. rnaintained irrespective of dispersion on ACF. The peaks C and D are caused by the 1 s 4 p transition associated with ligand-to-metal charge transfer.We notice that CuO has slightly different absorption band structure from others in the C-D region. XANES spectra, therefore, suggest the formation of Cu(OH),-like substances on ACF. Fig. 6 shows Fourier transforms (without phase-shift correction) of k3x(k) determined from the EXAFS oscillation x(k) for CuO, Cu(OH), and Cu(OH),-ACF. There are two main peaks [A (0.13-0.23 nm) and B (0.25-0.30 nm)] which are ascribed to Cu-0 and Cu-Cu distances, respectively. Although all Fourier transforms have similar features, the distance of peak B for CuO is shorter than that of other compound, but Cu(OH), and Cu(OH),-ACF have similar peak position. EXAFS data also suggest the formation of Cu(OH),-like substance on the ACF. Furthermore, the intensity of peak B for Cu(OH),-ACF is rather small than that for Cu(OH),.This indicates that the coordination number of Cu-Cu for Cu(OH),-ACF is smaller than that for Cu(OH), crystals; the CufOH), particles are highly dispersed on the ACF (especially on PIT). NO Adsorptivity Adsorption isotherms of NO on Cu(OH),-ACF are shown in fig. 7. The amount of NO absorbed on Cu(OH),-PIT at 80 kPa is greater than that on Cu(OH),-CEL. Cu(OH),-ACF exhibits a remarkable hysteresis and has great amount of irre- versible NO adsorption [80 mg g-l for Cu(OH),-PIT and 60 mg g-l for Cu(OH),-CEL, respectively]. Almost all the NO molecules are adsorbed in the micropores, as shown by the results of N, adsorption after preadsorption of NO in a previous paper.ll Temperature Dependence of Magnetic Susceptibility Fig.8 shows the temperature dependence of the magnetic susceptibility of gaseous NO, Cu(OH), and CuO powders. NO gas has a large positive value which decreases with1802 NO Dimers in Cu(OH),-ACF Micropores 16.0 14.0 12.0 7 10.0 00 s E ," 8-0 2 x" 6.0 4.0 2.0 \ 50 40 3 0 Fig. 8. The temperature dependences of the magnetic susceptibility of gaseous NO (O), Cu(OH), (0) and CuO (A). temperature, agreeing with the literature data.12 Cu(OH), powder also shows paramagnetic behaviour, coinciding with the data by Pierre and Gauthier.,l Heating Cu(OH), above 373 K in vacuo gives CuO; the magnetic susceptibility of Cu(OH), at 423 K drops to that of CuO, as shown by the broken line in fig. 8. The magnetic susceptibility of CuO is smaller than that of Cu(OH), and is almost independent of temperature.The temperature dependence of the magnetic susceptibilities of Cu(OH),- CEL with and without adsorbed NO is shown in fig. 9. Cu(OH),-CEL is diamagnetic and its magnetic susceptibility is negative and independent of temperature. The observed value agrees with the magnetic susceptibility calculated from the x values of CEL and 1 wt % Cu(OH),. Although the magnetic susceptibility data of the NO-absorbed ACF are slightly scattered, we can see a definite tendency. The magnetic susceptibility of Cu(OH),-CEL with adsorbed NO near room temperature is in the diamagnetic region, and larger than that of Cu(OH),-CEL without adsorbed NO. It increases sharply to a paramagnetic maximum value at 373 K and then decreases to a negative value again.The change in the magnetic susceptibility of Cu(OH),-CEL with temperature is much more remarkable than that of unmodified CEL reported ea~1ier.l~ For Cu(OH),-PIT, the fine Cu(OH), particles are not uniformly disperse on the ACF surface, because the magnetic susceptibility of Cu(OH),-PIT varies widely for each specimen; note that the Cu content in Cu(OH),-PIT differs from specimen to specimen (1 -2 f 0.5 wt %). However, a clear difference of magnetic susceptibility between Cu(OH),-PIT with and without adsorbed NO can be observed, as shown in fig. 10. Cu(OH),-PIT is also diamagnetic. The relationship between the magnetic susceptibility and temperature of Cu(OH),-PIT with NO has a similar peak around 373 K to that of Cu(OH),-CEL.The temperature dependences of both specimens are similar, although their absolute x values are different. Hence we consider that the same phenomenon as that in Cu(OH),-CEL takes place in Cu(OH),-PIT.K . Kaneko et al. 1803 03 0.2 0.1 0 7 -0.1 g -0.2 M 1 rg 2 . -0.3 x" -0.4 -0.5 -0.6 -0.7 T \ 3.2 2.0 2.4 2.0 300 3 50 4 00 T/K Fig. 9. The temperature dependences of the magnetic susceptibility x of Cu(OH),-modified CEL with (0, A, 0) and without (e) adsorbed NO and the temperature dependence of the x value calculated (---) from the x values of both the NO gas of the same amount as the adsorbed NO and the CEL. t 300 350 400 TIK Fig. 10. The temperature dependences of the magnetic susceptibility of Cu(OH),-modified PIT with (0, 0, A) and without (a) adsorbed NO.1804 NO Dimers in Cu(OH),-ACF Micropores - 4.0 - 5.0 -6.0 % E -7.0 - 8.0 -9.0 1 I i 2.5 3.0 3.5 103 KIT Fig.11. The van't Hoff plots of the dissociation constant of the NO dimer in the micropores of Cu(OH),-modified CEL. Dimeric NO in Micropores The magnetic susceptibility of Cu(OH),-ACF with adsorbed NO is negative near room temperature, irrespective of the adsorption of NO which has a large paramagnetic susceptibility. The temperature dependence of the x value calculated from the x values of both gaseous NO (the same amount as adsorbed NO) and CEL is shown by the broken line in fig. 9 for comparison. The calculated temperature dependence is clearly different from the observed dependence; it is evident that adsorbed NO does not exhibit paramagnetism.The negative x of Cu(OH),-ACF with adsorbed NO arises from the dimerization of NO in the micropores as well as in the case of unmodified ACF. The magnetic susceptibility data on gaseous NO and Cu(OH),-ACF, with and without NO, show that 98% of NO molecules in the micropores at 298 K are dimerized. The previous studyg by temperature-programmed desorption of Fe-ACF with adsorbed NO told us that adsorbed NO begins desorbing above 313 K. The NO dimers in the micropores dissociate and desorb with temperature according to eqn (2), giving rise to an increase in the magnetic susceptibility below 373 K: (NO), + 2 NO. ( 2 ) We can determine an equilibrium constant K in mole fractions from the magnetic susceptibility data in the temperature range 298-373 K : The In K us.1/T plots for Cu(OH),-CEL are shown in fig. 11. These plots are linear, although not all the lines overlap. The dissociation enthalpy, Ha, for the NO dimer is determined from the slope. The Ha values obtained are 21 f 3 kJ mol-' for Cu(OH),- CEL and 18f2 kJ mo1-l for Cu(OH),-PIT. The Hd values for NO molcules on Cu(OH),-ACF are larger than those of the condensed phase (6-12 kJ mol-l) in the literature,22,23 but the Hd values of Cu(OH),-ACF are slightly lower than those of unmodified ACF (24 kJ mol-' for CEL and 22 kJ mol-' for PIT);24 ultrafine Cu(OH), particles appear not to contribute to the stabilization of the dimer in spite of their acceleration of NO adsorption. The micropore width is ca. 0.9 nm, corresponding toK. Kaneko et al. 1805 triple layers of NO.The NO molecules in the slit-type micropores are stabilized both by the micropore field and the dimerization. We reported that the saturated vapour pressure of NO in the micropores at 303 K determined by the modified DR plot is ca. 110 kPa;'l the saturated vapour pressure of NO is probably that of the dimerized NO. The dimerization of NO must be associated with the enhancement of the micropore filling of supercritical NO. However, we do not understand as yet what role the fine Cu(OH), particles play in NO dimerization. The decrease in the magnetic susceptibility above 373 K is brought about the consumption of paramagnetic NO and production of diamagnetic CO, and N, through the reaction of darbon with NO: C+2NO = CO,+N,. (4) This reaction was discovered 58 years ago by Shah25 and was recently ascertained by Mochida et a1.26 and by oursel~es.~ Although we assume that the NO dimer has the cis-type structure, ON-N0,27 we have no spectroscopic evidence as yet.Studies on the molecular structure of the NO dimer in micropores are very important in ascertaining the adsorbate-adsorbate interactions enhanced by the micropore field. Special thanks are due to Prof. I. Taguchi for the SE micrographs. The partial financial assistance of Osaka Gas Company is gratefully acknowledged. References 1 M. M. Dubinin, Chem. Rev., 1960, 60, 235. 2 D. H. Everett and J. C. Powl, J. Chem. Soc., Faraday Trans. 1 , 1976, 25, 59. 3 S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity (Academic Press, London, 2nd 4 H.Marsh, Carbon, 1987, 25, 49. 5 P. J. M. Carrott, R. A. Roberts and K. S. W. Sing, Carbon, 1987, 25, 59. 6 B. McEnaney, Carbon, 1987, 25, 69. 7 J. H. de Boer, The Dynamical Character qf Adsorption (Clarendon Press, Oxford, 1968), p. 147. 8 K. Kaneko and K. Inouye, Carbon, 1986, 24, 772. 9 K. Kaneko, Langmuir, 1987, 3, 357. edn, 1982), p. 153. 10 K. Kaneko, Characterization of Porous Solids, Proc. IUPAC Symp. (Elsevier, Amsterdam, 1987), 11 K. Kaneko, T. Ohta, S. Ozeki, N. Kosugi and H. Kuroda, Appl. Surf. Sci., in press. 12 A. L. Smith and H. L. Johnston, J. Am. Chem. SOC., 1952, 74, 4696. 13 W. J. Dulmage, E. A. Meyers and E. L. Lippert, Acta Crystallogr., 1961, 14, 1100. 14 J. Billingsley and D. B. Callear, J. Chem. Soc., 1971, 589. IS K. Kaneko, N. Fukuzaki and S. Ozeki, J. Chem. Phys., 1987, 87, 776. 16 T. Sugimoto, N. Shindo and K. Tai, Sen-i-Kogaku, 1987, 40, 153. 17 K. S. W. Sing, D. H. Everett, R. A. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. 18 F. Rodriguez-Reinoso, J. M. Martin-Marinez, C. Prado-Burguette and B. McEnaney, J. Phys. Chem., 19 N. Kosugi, T. Yokoyama and H. Kuroda, Chem. Phys., 1984, 91, 249. 20 N. Kosugi, T. Yokoyama, K. Asakura and H. Kuroda, Chem. Phys., 1986, 103, 101. 21 P. Escoffier and J. Gauthier, C.R. Seances Acad. Sci., 1961, 252, 271. 22 E. A. Guggenheim, Mol. Phys., 1966, 10, 401. 23 C. E. Dinerman and G. E. Ewing, J. Chem. Soc., 1970, 53, 626. 24 K. Kaneko, N. Fukuzaki, T. Suzuki and S. Ozeki, J. Phys. Chem., to be submitted. 25 M. S. Shah, J. Chem. SOC., 1929, 2661; 2676. 26 I. Mochida, M. Ogaki, H. Fujitsu and Y. Komatsubara, Fuel, 1985, 64, 1054. 27 A. E. Enault and Y. Larher, Surf. Sci., 1977, 62, 233. in press. Siemieniewska, Pure Appl. Chem., 1985, 57, 603. 1987, 91, 515. Paper 711941 ; Received 2nd November, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401795

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chern. Soc., Faraduy Trans. I , 1988, 84(6), 1807-1816 The Association and Solvation of Formamide in Pyridine and Picolines Prem P. Singh Chemistry Department, Maharshi Dayanand University, Rohtak 124001, India Topological investigations of the volumetric and enthalpic effect in mixtures of formamide (A) and pyridine or a-, p- or y-picoline (B) have indicated that the state of association of (A) in an (A+B) mixture is dictated by the molecular entity B and that the magnitude of the attractive B-B interactions in the various picolines (B) varies in the order j? z y > 01. The energetics of the solution process characteristic of these (A-B) mixtures have also been investigated. The results obtained are rationalized by graph-theoretical arguments. Amides are compounds of considerable biological interest.' Spectroscopic and n.m.r.studies2 of the reactivity of the amide group have suggested that the unshared electron pairs of the carbonyl oxygen of the amide group are the positions of hydrogen bonding with other species. Further, in view of the two well known resonance structures of the arnide group3, it is reasonable to infer that the lower amides may exist, like alkan01s,~-~ as dimers, trimers or higher r-mers in the liquid state. However, Daviesg maintains that the association of amides in solution is limited to dimer formation. On the other hand, the nitrogen atom in pyridine is a n-electron acceptor," and is known" to be involved in a hydrogen- bonded interaction with chloroform. The addition of pyridine or picolines (B) to an amide, such as formamide (A), may thus cause rupture of the self-association in formamide, followed by its subsequent solvation in B.Further, since the electron density'(' at C,, C, and C, in pure pyridine is Iess than that in pure benzene, it is reasonable to infer that some kind of pyridine-pyridine or picoline-picoline interactions may characterize even the pure pyridine or picolines. Following Kier and Ha11,12 Singh et al. l3 have recently employed graph-theoretical arguments to investigate the association and solvation of A in A-B mixtures where A is ROH (R = CH, or C2H5) and B is CHX, (X = C1 or Br) or CH2Br2. Volumetric and enthalpic effects in A+B mixtures where A is formamide and B is pyridine or a-, p- or y-picoline may yield valuable information on the association and solvation of A in B in these (A+B) mixtures.Experimental Materials and Methods Formamide, pyridine and the a-, p- and y-picolines were purified by standard procedures. 14, l5 The purities of the purified samples were checked by measuring their densities at 298.15 kO.01 K, and these agreed to within 5 x g cm-' with their corresponding available literature Molar excess volumes, V E , were determined in a V-shaped dilatometer18 in a manner described elsewhere." The temperature of the water bath was controlled to within +0.01 K and the change in the liquid level of the dilatometer capillary was measured with a cathetometer that could read to & 1 mm. The uncertainty in the measured VE values is ca. 0.5%. Molar excess enthalpies, HE, for the various mixtures were measured at 298.15 K in 16, l7 as is shown in table 1.18071808 Solvation of Formamide in Pyridine and Picolines Table 1. Comparison of the measured densities at 298.15 & 0.010 K of the various compounds with their corresponding literature values density/g cm-3 compound this work literature ref. (1) formamide 1.12921 1.129 18 16 (3) a-picoline 0.939 80 0.939 82" 17 ( 5 ) y-picoline 0.95045 (2) pyridine 0.977 57 0.977 6 11 (4) Q-picoline 0.95248 - - - - a Evaluated from the density us. temperature plot. a flow microcalorimeter (LKB Broma, Sweden) in the manner described previo~sly.'~ The performance of the calorimeter was checkedl9 by measuring HE for benzene (A)-tetrachloromethane (B) mixtures, and these agreed to within the experimental uncertainties with the available literature values.The uncertainty in the measured HE values is ca. 1 YO. Results The measured VE and HE data recorded in tables 2 and 3 at 298.15 K for the various (A-B) mixtures were fitted to the expression 2 XE(X = Y or H ) = x, xB C x'")(2xA - I)" n-0 where (n = 0-2) are adjustable parameters and x, is the mole fraction of A in the (A-B) mixture. These parameters were evaluated by fitting XE ( X = V or H ) / x , xB to eqn (1) by the least-squares method and are recorded, together with the standard deviation of XE, a(XE), in table 4. The choice of n to have three distinct values (n = 0-2) was dictated by the consideration that the maximum deviation, am(XE), of XE [calculated from eqn (l)] from the corresponding experimental value satisfied the relation am(XE) < 2a(XE).Discussion We are unaware of any VE or HE data at 298.15 K for the present mixtures with which to compare our results. VE and HE are large and negative for all the present (A-B) mixtures, and for an equimolar mixture vary in the order pyridine > y-picoline > P-picoline > a-picoline. The increased negativities of VE and HE values when pyridine is replaced by picoline in their binary mixtures with formamide suggests' that in the competition between A-A and A-B interactions, the latter seem to be substantially favoured. This would then mean that as the introduction of pyridine or a-, p- or y- picoline (B) to formamide (A) would cause structural changes in A, so that it would be of interest to analyse VE data for the present (A-B) mixtures in terms of an approach20 that employs the graph-theoretical concept of molecular connectivity parameters of the third degree,12 35, of the constituents of these mixtures.According to this approach,20 VE for a binary mixture is given by 1 i=A, B i - A , BP. P. Singh 1809 Table 2. Measured VE values at 298.15 K for the various (A-B) mixtures as a function of the mole fraction xA of component A v'E/Cm3 VE/cm3 X A mol-1 X A mol-' formamide (Abpyridine (B) 0.0672 -0.054 0.5500 -0.1 16 0.0892 - 0.065 0.6001 - 0.108 0.1028 -0.072 0.6492 -0.097 0.1198 -0.081 0.7118 -0.082 0.1498 -0.098 0.7214 -0.080 0.2027 -0.115 0.7500 -0.074 0.2685 -0.130 0.8409 -0.051 0.3456 - 0.135 0.8804 -0.037 formamide (A)-a-picoline (B) 0.0648 -0.234 0.3803 -0.627 0.0752 -0.262 0.4923 -0.593 0.1249 -0.389 0.5568 -0.552 0.1864 -0.504 0.6536 - 0.475 0.2250 -0.556 0.6654 -0.465 0.2498 -0.575 0.6741 -0.455 0.2614 -0.588 0.8877 -0.194 0.3623 -0.627 0.9014 -0.173 formamide (A>-B-picoline (B) 0.0645 - 0.084 0.4569 -0.276 0.0851 -0.110 0.5148 -0.266 0.1 101 -0.136 0.6250 -0.230 0.1678 - 0.187 0.7227 - 0.182 0.2396 - 0.233 0.7550 - 0.162 0.4492 -0.276 0.8185 -0.126 formamide (A)-y-picoline (B) 0.0685 -0.101 0.5684 -0.220 0.0814 -0.118 0.6036 -0.206 0.1201 -0.158 0.6498 -0.187 0.2764 -0.253 0.6610 -0,180 0.3034 -0.260 0.7123 -0.154 0.3642 -0.267 0.7450 -0.140 0.4464 -0.259 0.7956 -0.112 0.5142 -0.240 0.8034 -0.107 0.5316 -0.235 where x, is the mole fraction of A and etc.are defined2' by In eqn (3) Sp etc. reflects21a explicitly the valency of the Zth etc.vertex in the molecular graph of A in forming bonds, and is related21* to the maximum valency, Z,, and the number of hydrogen atoms, h,, attached to the Zth etc. vertex by the equation Sy = Z , - h , . aAB in eqn (2) is a constant, characteristic of the (A-B) mixture. Since formamide (A) is expected to undergo structural changes in its binary mixtures1810 Solvation of Formamide in Pyridine and Picolines Table 3. Measured H E values at 298.15 K for the various (A-B) mixtures as a function of the mole fraction x, of component A x, HE/J mol-1 x, HE/J mol-l formamide (Akpyridine (B) 0.1635 -326.2 0.5128 -847.8 0.2815 -557.6 0.6785 -815.4 0.3055 -581.9 0.8003 -629.0 0.4470 -798.5 0.8191 -586.9 0.4686 - 817.2 0.8523 - 503.9 0.4980 - 839.0 0.9103 - 332.4 formamide (A)-a-picoline (B) 0.1678 -909.2 0.5479 - 1536.4 0.2917 - 1338.0 0.7233 - 1131.0 0.3122 - 1390.2 0.8310 -781.2 0.4979 - 1570.0 0.8616 -651.9 0.5 104 - 1566.4 0.8664 - 632.7 0.5206 - 1558.8 formamide (A)-/?-picoline (B) 0.1659 - 564.3 0.5446 - 1148.6 0.2889 -904.4 0.7206 - 892.4 0.3093 -951.8 0.8291 -590.4 0.4949 - 1 160.0 0.8324 - 58 1.6 0.5071 - 1160.3 0.8600 -491.6 0.5173 - 1159.3 0.8648 -475.4 formamide (A)-y-picoline (B) 0.1915 -660.9 0.7023 -955.8 0.3220 -938.1 0.8234 -690.3 0.3477 -976.6 0.8459 -620.8 0.5166 - 1097.4 0.8662 -558.4 0.5458 - 1094.9 0.9248 - 341.5 0.5605 - 1090.7 0.9292 - 323.6 with pyridine (B) or the picolines (B), '<A of A in an (A-B) mixture will not be the same as that in the pure state. Consequently, eqn (2) may be expressed1' in the form 1 (4) i-A, B where and 'ti denote, respectively, the 'c value of i in the mixture and in the pure state. Again, as the degree of association in pure A is not known with certainty and as no theoretical method is available to evaluate of A in the (A-B) mixture, we regarded and 'lB as adjustable parameters and evaluated them by fitting the experimental VE values to eqn (4).Only those and 'ti values were retained that best reproduced the experimental VE data, i.e. for which the variance of fit, p, defined by where ( q - p ) is the number of degrees of freedom, was a minimum. Such ('ti, i = A or B) values, together with the VE values calculated in this manner at various x,, are (3cB)m, P = c ( E p t l - e l c d 2 / ( q - P )P.P. Singh 181 1 Table 4. Comparison of V" and HE values [calculated from eqn (4), (19) and (21), (see text)] with their corresponding experimental values at 298.15 K for the various (A-B) mixtures as a function of the mole fraction of A, xA; also included are the interaction energies xiB, ;s and the parameters Fn) (X = V or H, n = 0-2) of eqn (1) together with the standard deviation a(XE) of XE property" 0.1 0.2 0.3 0.7 0.8 0.9 formamide (A)-pyridine (B) I/ (calcd) -0.089 -0.114 -0.133 -0.122 -0.086 -0.030 I' (exptl)" -0.080 -0.1 16 -0.132 -0.080 -0.060 -0.030 If (calcd) -178.7 -385.0 -581.3 -802.9 -643.1 -377.6 If (exptl)b -200.0 -400.0 -581.0 -810.0 -637.0 -360.0 V0) = -0.496, V(I) = 0.2798, V2) = -0.1293; a(V") = 0.001 cm3 mole', aAB = 0.5619; WO) = - 3360.0, H ( l ) = - 1197.9, Hc2) = 392.4; a(H") = 0.8 J mol-'; (3<A)m = 0.32, = 0.33; (:3rR)m = 0.64, 3(B = 0.60; xi, = -746.1 J mol-'; x = - 7422.1 J mol-'. formamide (Aka-picoline (B) V (calcd) -0.503 -0.551 -0.583 -0.490 -0.384 -0.225 F.' (exptl)" -0.375 -0.535 -0.610 -0.430 -0.310 -0.175 H (calcd) -641.4 - 1100.5 - 1397.9 - 1254.1 -934.1 -514.0 H (exptl)b -600.0 - 1020.0 - 1360.0 - 1240.0 -920.0 -500.0 V(O) = -2.360, V ( l ) = 1.0715, VC2) = -0.7263; a( V") = 0.002 cm3 mol-', aAR = 2.655; W0) = -6280.0, H(') = 714.29, H ( 2 ) = 559.53; a(HE) = 1.07 J mol-l; (3cA)m = 0.60, = 0.602; (3'5R)m = 1.2; 3(r, = 1.0; xir, = - 3704.72 J mol-'; x = - 1927.8 J mol-I.formamide (A)-p-picoline (B) V (calcd) -0.139 -0.229 -0.253 -0.230 -0.127 -0.106 V (exptl)" -0.115 -0.210 -0.258 -0.196 -0.133 -0.067 H (calcd) -386.6 -704.5 -944.6 - 1006.4 -775.7 -440.9 V (exptl)" -340.0 -660.0 -940.0 -980.0 -720.0 -400.0 V0) = - 1.080, (3(R)m = 1.1, 3cB = 1.0; xiB = -2087.24 J mol-'; x = -4077.7 J mol-'.Vcl) = 0.3869, Vc2) = - 0.0208 ; = 1172.63 ; a(HE) = 0.87 J mol-'; (3cA)m = 0.60, a( V") = 0.001 cm3 mol-' ; aAB = 1.7213 ; = 0.603; = -4640.0, H ( l ) = 59.53, formamide (A)-y-picoline (B) V (calcd) -0.180 -0.214 -0.238 -0.180 -0.113 -0.048 V (exptl)" -0.127 -0.210 -0.258 -0.160 -0.110 -0.055 H (calcd) -369.8 -670.9 -896.2 -943.3 -725.6 -411.7 H (exptl)b - 340.0 - 660.0 - 900.0 - 960.0 - 760.0 -430.0 Vco) = -0.980, Vcl) = 0.5893, V2) = -0.1399; a(VE) = 0.001 cm3 mol-'; aAB = 1.8018; H(O) = -4379.99, = -357.16, W2) = -303.53; a(H") = 1.1 J mol-'; (3(A)m = 0.60, = 0.63 ; (3<B)m = 1.2, 3(R = 1.1 ; xiB = - 2005.5 J mol-'; x = - 4039.0 J mol-'.a Y and H denote V" and H E , respectively. are in J mol-'. Read from XE (X = V or H) us. x, plots. Vn) (n = 0-2) are in cm3 mol-l; while H(n) (n = 0-2) recorded in table 4. Since the agreement between the experimental and calculated VE values is reasonably good, the present 34 and (3<i)m values can be relied upon to yield meaningful information about the state of A and/or B in these (A-B) mixtures. Such an analysis13 of VE data for mixtures where A is ROH (R = CH, or C,H,) and B is CHX, (X = Cl or Br) or CH,Br2 in terms of eqn (4) has yielded information about the state of aggregation and solvation of A that is consistent with the information provided by an independent study2, based on an ideal associated approach for their HE and ac tivit y-coefficien t data. 60 FAR 11812 Solvation of Formamide in Pyridine and Picolines # but also that (,tB), # 3<B in all these (A-B) mixtures.Since ,< values of a constituent depend on the 6" values of the various vertices of its molecular graph, the present analysis suggests that both A and B are undergoing some kind of changes in their topology. An examination of the (3<)m values in table 4 shows not only that In our analysis we assumed that formamide in the pure liquid state exists as H\3.0 5.5 3.0,c=o- -- H 1.5 \A5 3.0 N - H-N \ . 5.5 3.0/ "o=c I H ( I ) or or 3.0~05.5 3.0, I HzN c,H In states (I) and (111) the oxygen atom of the carbonyl group is involved in hydrogen- bonded interactions.In the absence of a hydrogen-bonded interaction the four non- bonded electrons and the two on the =O fragment of the carbonyl group of formamide require21b that the maximum value of 6' (0) is 6. This means that 6" (0) (= 2- h) in (I), (11) and (111) must be between 5 and 6. Similarly, the 6" for the hydrogen atom involved in this hydrogen-bonded interaction would lie between 1 and 2. Following our earlier work', we arbitrarily assigned 6" (0) = 5.5 and 6" (H) = 1.5 to those oxygen and hydrogen atoms that are involved in the hydrogen-bonded interaction. The dotted line in (11) denotes that there is charge delocalization between the N and 0 atoms (which is consistent with spectroscopic studies2,, 24 on amides) spilling over onto the hydrogen atoms of the -NH, group.The 6" value for the carbon atom in the ECH configuration was taken21 to be 3. The values of various vertices in the formamide backbone are shown in (1)-(111). The ('ti), values for configurations (I)-(111) of A were then calculated [from eqn (3)] to be 0.58, 0.312 and 0.434, respectively. As the value of in formamide (A)-a-, or p- or y-picoline (B) mixtures is close to 0.6 (table 4), the values of (I), (11) and (111) suggest that in all these mixtures formamide in the pure state exists mainly as cyclic dimers (I), with perhaps a proportion of configuration (11). On the other hand, a (3tA)m value of 0.33 (table 4) for formamide (A) in pyridine mixtures suggests that here formamide exists mainly as monomer (11) [with Further ab-initio molecular-orbital calculations" on the structural, energetic and electronic properties of p- and y-R-pyridines (R = CH,, NH,, OH, F etc.) have SuggestedlO that the n-electron density at the C,, C, and C, positions in pyridine is less than that in benzene.This suggests that either one, or two or all three electron-deficient carbon centres in one pyridine molecule are involved in weak interactions with the n-electron cloud of the other pyridine molecule. We next assumed that the electron- deficient C, centre in one pyridine molecule is involved in interaction with the n-electron system of the other pyridine molecule, and then evaluated ,<' for pyridine for configuration (IV) assuming that P(n) = 1. Such a procedure yielded ('<;), = 0.706 for configuration (IV) of pyridine.[The 6" values of the various vertices are also shown in (IV) and the 6" value for the nitrogen atom of pyridine was calculated in the manner suggested by Kier.21] This value for pyridine is very close to the value (3c5B)m = 0.6 (table 4) obtained from an analysis of VE data of formamide (A)-pyridine (B) mixtures in terms of eqn (4). Further, as the equilibrium geometry of the pyridine ring has been shownlO to be independent of the nature and position of the substituents, it follows that a similar scheme of molecular interactions should also characterize picolines. Consequently, we next evaluated ('<;), configuration values for (V), (VI) and (VII) configurations of = 0.3121.P. P. Singh 1813 3.0 3.0 5.0 5.0 3 . 0 0 ‘ 0 3.0u N 4 o 1.0 3.ou N- < I N5.0 3.0 Is-.’ 3.0 3.0 ‘.-el 3.0 3.0 \--‘ 4.0 ‘-O ‘,-, , 3.5 I3.5 13.5 ’.O 3.0 ,3.5 I I I I 5.0 I I I Ip P m a, D- and y-picoline, respectively, again utilizing P(n) = 1.This yielded ( 3 c 3 m values of 1.012, 1.371 and 0.933 for a-, p- and y-picoline, respectively, These (3(& values are close to the corresponding (3c& values of 1.0, 1.0 and 1.1 (table 4) obtained from an analysis of their VE data with formamide in terms of eqn (4). in formamide (Aka-, p- or y-picoline (B) mixtures, and since formamide has been postulated to exist in configuration (I) and/or (11), it is reasonable to assume that only a small part of the configuration of (A) contributes to the 3tB value of B in all these (A-B) mixtures. If it is now postulated2 that the hydrogen atom attached to the -C=O group of (A) is involved in hydrogen bonding with the nitrogen atom of pyridine (B) or picoline (B), then the molecular entity that should determine the ’( value of B in all these (A-B) mixtures should be (VIII), (IX) or (X) with R = H or CH,.Postulating molecular entity (VIII) in these mixtures would then yield (3~&, values of 0.783, 1.085, 1.200 and 1.079 for pyridine and a-, p- and y-picoline, respectively. On the other hand, (3(b)m values for pyridine and a-, p- and y-picoline (B) would be 1.375, 1.775, 2.057 and 2.105 for the molecular entity (IX) and 1.296, 1.850, 1.631 and 1.459 for the molecular entity (X). It is thus evident that the (3&Jm values evaluated for pyridine and a-, 8- and y-picoline (B) are very close to the corresponding (315B)m values of 0.6, 1.200, 1.100 and 1.20 (table 4) for the molecular entity (VIII) only.The present analysis thus suggests that all these mixtures are characterized by the presence of molecular entities having the configuration (VIII). We next undertook a study of the energetics of the various interactions characterizing these mixtures. If we assume what has been stated above to be reasonable, then the process of the present (A-B) mixture formation would require (a) a mixing of (A) with (B) to establish (A)-(B) contacts with an interaction energy xAB per mole of (AHB) contacts. (b) these (A)-(B) contacts between (A) and (B) would then cause rupture of (i) the intramolecular association in A to yield monomers and (ii) the intermolecular interactions in B to yield ‘free’ B molecular entities.(c) The monomers of A would then undergo solvation in ‘free’ B molecules to form AB molecular entities. Consequently, if AH, is the molar enthalpy change due to process (a) then the enthalpy change AHl due to process (a) would be given13 by In addition, since (3cA)m = AH1 = XA xAB S B ( 5 ) where SB is the surface f r a c t i ~ n ~ ~ , ~ ~ of B, defined by so that (7) On the other hand, if xAA is the energy per mole required to cause rupture of the self- association in A, and xBB is the corresponding energy per mole required to cause rupture 60-21814 Solvation of Formamide in Pyridine and Picolines R R R Ix of intermolecular interactions (B-B) in pure B, then the enthalpy changes AH2 and AH, due to processes (b) (i) and (b) (ii) would be given13 by an expression identical to expression ( 5 ) , i.e.by (8) (9) AH2 = XA xAA SL AH3 = XA xBB s;3 where S;3 is the surface fraction of B that brings about changes in A. Evidently Sb would depend13 on the mole fraction of A and also on the surface of B in the (A-B) mixture, so that sk x, S B (10) or Hence and where k and k' are constants. On the other hand, if x12 is the interaction energy per mole for process (c), resulting in the formation of AB molecular entities, then the enthalpy change, AHq, due to this process should be expressed18 by where k" is another constant of proportionality. The total enthalpy change, HE, due to processes (a), (b) and (c), resulting in the formation of the (A-B) mixture from pure A and B would then be given by 4 HE(T, xA) = C AHi i-1 = LxA xB 'B/ c (xi y)] k AR + kXAA xA + k'XBB xA +kNX12 xB)* (' 5, i - A , B Now [taking account of configurations (I) and (VIII)] ifP.P. Singh 1815 then eqn (1 5) yields H E ( T, However, VA/VB has been xA) = Ex, xR 'B/ c (xi %)I (2xaB + x A X ) (18) i = A , B taken13 equal to ( 3 c A / 3 r B ) , so that eqn (18) reduces to Evaluation of xiB and x (and hence xAB, xI2, xBB and x,,) would then require a knowledge of H E data for the (A-B) mixture at two mole fractions. For the present analysis we utilized H "( T, x, = 0.4 and 0.5) data for the (A-B) mixture to evaluate xiB and x [from eqn (19)] and these were subsequently employed to calculate HE data for the (A-B) mixture at another value of x,. Since processes (a), (b) and (c) of (A-B) mixture formation would apply [in view of configurations (I) (V) and (VIII)] strictly to formamide (A)-a- or p- or y-picoline (B) mixtures, we evaluated [from eqn (1 9)] HE data for these mixtures at various x,.Such H E data are recorded in table 4 and are also compared with their corresponding experimental values. Also recorded in table 4 are xiB and x interaction energies characteristic of the various formamide (A)-picoline (B) mixtures. Again, as formamide in formamide (Akpyridine (B) mixtures has been postulated to exist as a monomer [configuration (11)], the solution process for this mixture would involve process (a), (b) (i) and that stated above. Consequently, H E for this particular mixture would be given by which in view of eqn (7), (13), (14), (17) and (18) yields H E = AH, -I- AH3 -k AH4 (20) Evaluation of H E data [from eqn (21)] for formamide (Appyridine (B) mixtures would then, of necessity, require that H E data at two compositions be known.For the present analysis we utilized HE(T, x, = 0.4 and 0.5) data to evaluate xiB and x, which were subsequently employed to evaluate H E for the mixture at any x,. Such H E values at various x, along with xiB and x values characteristic of this mixture, are recorded in table 4. Examination of table 4 clearly shows that the HE data calculated from eqn (19) for formamide (A)-a-, a- or y-picoline (B) mixtures and from eqn (21) for formamide (A)-pyridine (B) mixtures compare well with their corresponding experimental values. Further, since xiB = kxAA = k"XIz = xAB [eqn (16)] and x = k'x,, [eqn (17)], and if it is assumed that k' is the same for all three picolines (B) in the present (A-B) mixtures, then the xBB values in table 4 show that B-B interactions among the various picolines are attractive and that their strength increases in the order p x y > a.This is understandable : since the B-B interactions in configurations (V)-(VIII) of the various picolines are due to charge rearrangements their magnitudes should be determined by the hyper- conjugative effect of the methyl group within the molecule. It is known2' that the magnitude of the hyperconjugative effect of the methyl substituent in a-, p- and y- picoline (B) varies in the order a > y x p, so that the B-B interaction should be largest in a-picoline and smallest in a-picoline.This would further require that the boiling points of these picolines should vary in the order x y > a ; their actual boiling points28 support such a conclusion. Again, as the introduction of a methyl substituent into the pyridine molecule should increase the availability of the lone-pair electrons on the nitrogen atom (owing to the inductive effect of the methyl group] and as the base strength of pyridine and a-, p- and y-picoline increases27 in the order pyridine1816 Solvation of Formamide in Pyridine and PicoIines < y- c p- < a-picoline, it follows that the energy released when pyridine or a-, p- or y-picoline forms hydrogen-bonded entities (VIII) with formamide should vary in the order pyridine < y- < p- c a-picoline.If k” in eqn (16) is assumed to be the same for pyridine and a-, a- and y-picoline, then the xiB values for the various (A-B) mixtures in table 4 clearly show that this is true. The present study thus has indicated that the state of association of a formamide in forrnamide (A)-pyridine or a-, p- or y-picoline (B) mixtures is determined by the molecular entity B and that the volumetric and enthalpic effects in (A-B) mixtures can be rationalized by graph-theoretical arguments to yield information that is consistent with the more involved ab-initio molecular-orbital calculations. It has provided an insight into the energetics of the formation of A-B solution from the pure components A and B. I thank the Head of the Chemistry Department, and the authorities of Maharshi Dayanand University, Rohtak for providing the necessary research facilities.References 1 P. Assarsson, N. Y. Chen and F. R. Eirich, in Colloidal Dispersions and Micellar Behaviour, ed. K. L. 2 C . C. Costain and J. M. Dowling, J . Chem. Phys., 1960, 32, 158. 3 L. Pauling, in The Nature ofthe Chemical Bond (Cornell University Press, 3rd edn, 1960), p. 281. 4 F. Franks and D. J. G. Ives, Q. Rev. Chem. SOC., 1966, 20, 1. 5 K. B. Whetsel and J. H. Lady, Spectroscopy of Fuels (Plenum, New York, 1970), pp. 259-279. 6 E. M. Woolley, J. G. Travers, B. P. Erno and L. G. Hepler, J. Phys. Chem., 1971, 75, 3591. 7 J. R. Johnson, S. D. Christian and H. E. Affsprung, J . Chem. Soc., 1967, 764. 8 J. S. Rowlinson, Liquids and Liquid Mixtures (Butterworths, London, 2nd edn, 1969), p.159. 9 M. Davies, in Hydrogen Bonding, ed. D. Hadzi and H. W. Thompson (Pergamon, New York, 1956), Mittal, ACS Symp. Ser. (American Chemical SOC., Washington, D.C. 1975), vol. 9, p. 288. p. 271. 10 J. E. Delbene, J. Am. Chem. SOC., 1979, 101, 6184. 11 T. J. V. Findlay, J. S. Keniry, A. D. Kidman and V. A. Pickles, Trans. Faraday Soc., 1967, 63, 846. 12 L. B. Kier and L. H. Hall, in Molecular Connectivity in Chemistry and Drug Research (Academic Press, 13 P. P. Singh, V. K. Sharma and S. P. Sharma, Thermochim. Acta., 1986, 106, 293. 14 A. Vogel, Practical Organic Chemistry (Longman Green, London, 3rd edn, 1973), p. 145. 15 J. A. Riddick and W. B. Bunges in Organic Solvents: Physical Properties and Methods of Purifzcation 16 J. Timmerman, Physico-chemical Constants of Pure Organic Compounds (Elsevier, Amsterdam, 1950), 17 R. Muller and H. Brenneis, Z . Electrochem., 1932, 38, 451. 18 P. P. Singh and S . P. Sharma, J . Chem. Eng. Data, 1985, 30, 477. 19 H. P. Dahiya, P. P. Singh and S. Dagar, Fluid Phase Equilibria, in press. 20 P. P. Singh, Thermochim. Acta, 1983, 66, 37; and references cited therein. 21 (a) L. B. Kier, in Physical Chemical Properties of Drugs, ed. S. H. Yalkowski, A. A. Sinkula and 22 P. P. Singh, B. R. Sharma and K. S. Sidhu, Aust. J. Chem., 1979, 31, 1419 and references cited 23 R. E. Richards and H. W. Thompson, J . Chem. Soc., 1947, 1248. 24 T. L. Brown, J. F. Regan, R. D. Schuet and J. Sternberg, J . Phys. Chem., 1959, 63, 1324. 25 M. L. Huggins, J . Phys. Chem., 1970, 34, 371. 26 M. L. Huggins, Polymer, 1971, 12, 389. 27 H. C . Brown and G. K. Barbaras, J . Am. Chem. SOC., 1947, 69, 1137. 28 CRC Handbook of Chemistry and Physics, ed. R. C. Weast, (CRC Press, Cleveland, 58th edn, 1977), New York, 1976). (Wiley-Interscience, New York, 3rd edn, 1970), pp. 595 and 839. p. 582. S. C . Valvani (Marcel Dekker, New York, 1980), chap. 9, p. 295; (b) p. 297. therein. p. c-474. Paper 71520; Received 23rd March, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401807

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. Soc., Furaday Trans. I , 1988, 84(6), 1817-1834 Analysis of Temperature-programmed Diffusion Chromat ograms obtained with Zeoli te-Gas Sys terns Dan Fraenkel" and Alon Levy Department of Materials Research, The Weizmann Institute of Science, Rehovot, Israel A complete theoretical solution of the diffusion equation for gas in uniform spherical particles under temperature-programmed conditions is provided. From this solution temperature-programmed diffusion (t.p.di.) curves have been computed for a wide range of heating rates, a, and activation parameters, E and Do. Although the entire t.p.di. curve differs markedly from the curve based on the first term of the sum in the diffusion equation (from which previous t.p.di. expressions were derived), it is shown that both curves lead to the same value of E, and the difference between the pre- exponential factors, Do, is very small.Computed maximal-rate temperatures are shown to obey perfectly the pseudo- Arrhenius linear dependence of log (TL/p) on 1 / Tv. Simple, easy-to-use peak-shape expressions relating the activation parameters to TM and the half-height width of the t.p.di. curve, w, are derived from the diffusion equation and computed quantities, i.e. 3.6RT2, 2.5P1-i E=- and Do=- exp (3.6TJw). w 7r2w The peak-shape method is compared with and concluded to be com- plementary to the graphical method. Experimental t.p.di. curves of hydrogen encapsulated in various forms of zeolite A agree fairly well with computed curves based on activation parameters calculated by either of the above methods from experimental values of TM and /? or u.Finally, the effect of particle non-uniformity is shown to be substantial and to require a compensation factor in the calculation of E (and Do). In practice, however, this effect is normally in the range of experimental inaccuracy, and thus can be ignored. The application of temperature-programmed desorption (t.p.d.) methods in sorption and decomposition processes has attracted considerable interest in recent ~ e a r s . l - ~ It has been also acknowledged that in porous materials diffusion, rather than desorption, may be rate-determining, and an attempt was made to treat this problem using t.p.d. principles.'j Previously, mathematical t.p.d. expressions were derived for gas diffusion in cylindrical pores,' rectangular particles8 and spherical particle^.^ In all cases only the first term in the sum of the diffusion equation was considered, and although it was roughly shown that by neglecting all other terms not much harm is caused to the final sol~tion,~ an attempt to justify this neglection convincingly does not seem to have been made so far.The importance of temperature-programmed diffusion (t.p.di.) was shown in the decapsulation of non-polar gases entrapped in zeolites. This system is especially convenient for measurements of activated diffusion, since in it sorption effects can safely be ignored. Furthermore, owing to the simplicity of the shape of small guest molecules and the structure of the zeolite host (a matrix of uniform pores and windows having a well defined geometry) the process of gas release from zeolite cavities can be perceived in terms of a pure single-jump event at which the molecule passes from one pore to the 18171818 Temperature-programmed Difusion in Zeolites other through a potential barrier located at the saddle point of the window interconnecting these pores.In this paper we present an accurate solution of the diffusion equation (for a gas in spherical particles) under temperature-programmed conditions, from which complete t.p.di. curves have been computed. A comparison between theory and experiment is provided. In view of the TM values obtained we examine the validity of the 'classical' t .p.di. equation previously empl~yed.~ We also develop a simplified peak-shape t.p.di. method, based in part on the presented solution, and discuss its usefulness vis ci vis that of the graphical method based on the 'classical' equation.The application of these two methods in comparing the diffusional behaviour of different gases in Cs,Na-A(gas) encapsulates as a function of window blocking is presented in the succeeding article." T.P,Di. Curve The diffusion equation for uniform constant, is Theoretical spherical particles, for a very large Henry's law where U is the fraction of gas diffused at time t, ro is the radius of the particle and = 1 D(t) dt D(t) being the diffusion coefficient related to the activation parameters, Do and E (2) through the equation D(t) = Do exp [ - E/RT(t)] where R is the gas constant. In t.p.di. we follow U or the diffusion rate, dU/dt (or dU/dT) as a function of t (or T).Differentiating eqn (l), we obtain a3 dU 6 - = -D,* exp(-E*/T) C exp(-n2z*) dt n2 n-1 (3) where DO* = Do/(ro/z)2,z* = z/(ro/n)2 and E* = E / R and, assuming linear change of T with t, i.e. a constant heating rate, /? = dT/dt, z* is given by the integral equation D,* T t z* = JTtx0 exp ( - E*/ T) dT. (4) Since for TIE* < 0.1 the integral in eqn (4) can be satisfactorily approximated by the asymptotic expansion l1 Tt exp (- E*/T)dT = T exp (--E*/T) (TIE*)"( - l)"-lm! (5) 6.. m-1 D* T it follows that z* = L e x p ( - E * / T ) C (T/E*)"(-l)"-'m! D m-1 or exp (- E*/T) (1 - A +;A2- 3A3 +$A* -$A5 + . . .) D,* T2 r* = ~ BE* where A = 2T/E*.D. Fraenkel and A . Levy 1819 Combining eqn ( 3 ) and (6) we finally obtain exp (- E*/ T ) C ( T/E*)*( - I)"-lm ! m - 1 (8) dU 6D,* 03 - dT-27 n-1 Peak-shape Expressions Let us first determine the inflection points of the differential t.p.di.curve obtained by neglecting all terms in the sum of eqn (1) having n > 1 and equating the third derivative of U with respect to t to zero. Thus The superscript 1 denotes restriction to the first sum-term in the diffusion equation. Introducing the first, second and third derivatives of zl* into eqn (9), and rearranging the obtained equation, we arrive at (D;*T;2)2 - 3/3E'*(Di* Ti2) + ypE'*2 = 0 (10) or A2 -3A + = 0 (1 1) where A = D:* T:2/&??1*, y = 1 -A; and A: = 2T:/E'*, 7': being the temperature at the inflection point and D: the corresponding diffusion coefficient relating to the activation parameters through an equation analogous to eqn (2).Assuming that y is roughly constant, eqn (1 1) can be considered a quadratic equation whose roots are Dividing one root by the other and rearranging, we obtain ( 1 3 ) where T:, and Ti- are the higher and lower inflection temperatures, respectively, AT: = Ti+-- T:-, and F, is given by the expression 2.3T:+ T:- AT! Fy El* = 3 + (9 - 4 y)i 3 - (9 - 4y)p F, = lg 1 - 2 lg v:+/ T;J and is assumed to be constant. Let us now introduce two additional approximations, i.e. T:, Ti- x pTg and AT: x q d , p and q being constants, to obtain By analogy, we assume the same form of expression for E*, (16) P = f - T& cu where f replaces (2.3pq/q). follows : D p can be obtained from the product of the roots of the quadratic equation as1820 Temperature-programmed Difusion in Zeolites T Fig.1. Typical t.p.di. curve and its first-sum-term component. ol/o = 2/3 (see table 4). Eqn (17) can be approximated to where p/yi is constant and again, by analogy, we can assume that in which the constant g replaces ply;. To express DO* in terms of co we introduce eqn (16) into (19) to get Pf DO* = - exp (fT,/o). g o Results and Discussion Computed T.P.Di. Curves T.p.di. curves were computed on the IBM 3081 mainframe of the Weizmann Institute using a FORTRAN program based on eqn (8) with n going from 1 to 20 and m from 1 to 10. From the program, maximal dU/dTand T(i.e. T,) values were obtained for the sum as well as its first five terms (n = 1,2, , .., 5), for given p, E* and D,* in the ranges 0.01 < P/K s-' < 0.64, 4000 < E*/K < 32000 and lo1 < D:/s-l < From the first-term curve, Ti, and Ti- were extracted. So were Tvalues corresponding to the half-maximum- intensity points of the sum and the first term, from which the respective half-height widths, co and 09, were obtained.As seen in fig. 1, the difference between the t.p.di. curve and its first-term component is rather pronounced, and is manifested mainly below T,, since higher-term contribution shifts appreciably to lower temperature with increasing n. &?--DO* combinations were chosen so as to give curves within a reasonable temperature interval, i.e. ca. 300-900 K. Representative results are listed in table 1 . The choice of E* and D,* has a clear consequence on the relation between the sum curve and its n-term components.This is illustrated in fig. 2. In the higher-energy case the curve is more asymmetric, the contribution of the first term near the peak maximum being largerD. Fraenkel and A . Levy 1821 2 00 2 50 3 00 1 I I , I , I , I , I- 0.015 0.010 0.005 3.000 460 480 500 520 540 560 T/K Fig. 2. Computed t.p.di. curves (sum and its first five terms). p = 0.16 K s-l. (-) E* = 32000, D; = 1 0 2 4 ; (----I E* = 4000, D,* = 104. 0,015 0 1 0.010 0.005 300 500 ? 700 L ' I ' - ' 1 ' ' " I " " 1 " " I " "1 1 800 T/K Fig. 3. Effect of D,* on computed t.p.di. curves. B = 0.16 K s-l. E* values: (a) 4000, (b) 8000, (c) 16000 and ( d ) 32000. Values of 1gD; are given next to each peak.1822 Temperature-programmed Difusion in Zeolites Table 1.Selected data of computed curves 3 4 6 8 1 2 4 8 16 32 64 2 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 3 2 4 8 16 32 64 4 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 7.26 6.52 5.84 5.20 4.62 4.1 1 9.16 8.3 1 7.5 1 6.75 6.05 10.1 13.4 12.3 11.3 10.4 9.45 8.58 17.3 16.1 14.9 13.8 12.7 11.7 7.26 6.70 6.17 5.66 5.18 4.72 9.30 8.65 8.04 7.45 6.88 6.34 14.2 13.4 12.6 11.8 11.1 10.4 20.3 19.3 18.3 17.4 16.5 15.6 E* = 4000 K 6.45 5.78 5.16 4.58 4.08 3.66 9.02 8.19 7.4 1 6.68 5.99 5.35 12.1 11.1 10.2 9.30 8.46 7.66 15.7 14.6 13.5 12.4 11.4 10.5 E* = 8000 K 6.57 6.05 5.56 5.09 4.65 4.23 8.44 7.85 7.28 6.74 6.2 1 5.72 13.0 12.2 11.5 10.8 10.1 18.6 17.7 16.8 15.9 15.1 14.3 9.47 388.0 410.6 435.9 464.3 497.2 537.3 327.0 343.4 361.4 38 1.2 403.1 427.4 281.5 293.9 307.4 322.0 337.9 355.4 246.6 256.2 266.6 277.7 289.8 302.9 540.1 563.1 587.9 614.7 644.0 675.9 475.1 493.1 5 12.4 533.1 555.5 579.7 381.4 393.2 405.7 419.0 433.1 448.2 3 17.5 325.8 334.5 343.6 353.3 363.4 403.4 428.3 456.3 488.2 525.4 571.5 337.2 354.9 474.4 395.9 420.0 446.9 288.7 301.8 316.2 331.8 349.0 367.9 251.8 261.9 272.9 284.7 297.5 31 1.4 553.1 577.4 603.7 632.3 663.6 697.9 484.8 503.6 523.9 545.7 569.3 594.9 387.3 399.5 412.4 426.2 440.9 456.6 321.4 329.9 338.8 348.3 358.2 368.7 132.2 148.0 166.9 190.2 221.2 94.2 103.8 114.9 127.6 142.7 160.4 70.1 76.4 83.4 91.5 100.5 111.1 53.9 58.2 62.9 68.2 74.3 81.0 129.2 140.3 152.8 166.8 182.9 201.2 100.2 108.0 116.4 125.9 136.6 148.6 64.9 69.0 73.4 78.2 83.5 89.3 45.1 47.5 50.1 52.8 55.8 59.0 -D.Fraenkel and A .Levy Table 1. (cont.) I823 __ - 6 8 12 16 14 17 20 24 (dU/dT),,,/10-3 K-' TM/K __-- p/ 10-2 K s-' c 1 st E 1st m/K 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 2 4 8 16 32 64 7.52 7.10 6.70 6.30 5.92 5.56 10.6 10.1 9.64 9.16 8.70 8.24 18.6 18.0 17.3 16.6 16.0 15.4 29.0 28.2 27.3 26.5 25.7 24.8 12.2 11.8 11.4 11.0 10.6 10.3 16.5 16.0 15.6 15.1 14.7 14.2 21.5 20.9 20.4 19.9 19.4 18.9 29.2 28.5 27.9 27.3 26.7 26.1 E* = 16000 K 6.88 740.6 6.49 762.8 6.12 786.4 5.75 811.4 5.40 837.9 5.06 866.2 9.78 619.3 9.3 1 635.1 8.85 651.6 8.40 669.0 7.97 687.3 7.55 706.6 17.2 464.5 16.6 473.5 16.0 482.8 15.3 492.5 17.7 502.6 14.2 513.0 26.9 370.4 26.1 376.2 25.3 382.1 24.5 388.3 23.7 394.6 23.0 401.1 E* = 32000 K 11.3 10.9 10.5 10.2 9.82 9.48 15.3 14.8 14.4 14.0 13.6 13.2 19.9 19.4 18.9 18.4 18.0 17.5 27.1 26.5 25.9 25.4 24.8 24.3 810.8 824.6 838.8 853.5 868.7 884.5 694.8 705.0 715.4 726.2 737.2 748.6 607.3 615.1 623.1 631.3 639.7 648.3 519.6 525.4 53 1.2 537.2 543.3 549.5 751.5 774.5 798.9 824.8 852.3 881.8 626.6 642.8 659.8 677.7 696.5 716.4 468.4 477.5 487.0 496.9 507.2 517.9 372.8 378.7 384.7 390.9 397.3 403.9 816.6 830.6 845.1 860.0 875.5 891.5 699.0 709.3 719.8 730.7 742.0 753.5 610.5 618.3 626.4 634.7 643.2 651.9 521.9 527.7 533.6 539.6 545.8 552.1 122.4 129.8 137.8 146.7 156.3 166.9 85.9 90.3 94.9 100.1 105.6 111.6 48.5 50.4 52.4 54.5 56.7 59.1 31.0 31.9 32.9 34.0 35.1 36.2 74.0 76.5 79.1 81.9 84.9 88.0 54.5 56.0 57.7 59.5 61.2 63.2 41.7 42.7 43.8 45.0 46.2 47.5 30.5 31.1 31.9 32.6 33.4 34.11824 Temperature-programmed Digusion in Zeolites I I I 0.0100 - 0.0075 M \ c 0.0050 z 0.0025 0.0000 200 300 400 500 600 0.015 - 0.010 - 0.005 - 300 400 500 600 700 TIK Fig.4. Effect of on computed t.p.di. curves. The respective p values, in K s-l, are given near each peak. (a) E* = 4000, D,* = lo2; (b) E* = 16000, D,* = lolo. 1.0 2 .o 3 . 0 I O3 KIT, 4.0 Fig. 5. Arrhenius-like plots from computed T, values. Indicated near each line are the respective E* and 1gD; values. Points are based on sum data taken from table 1. and that of the other terms smaller. On the other hand, at higher energy terms with large n contribute more at the far-left part of the sum curve, causing more tailing there. The influence of changing D,* at constant /? (i.e.0.16 K s-') is shown in fig. 3(a)--(d) and the effect of p for two typical E* - D,* combinations is illustrated in fig. 4(a) and (b). T.p.di. curves shift to higher temperature and broaden on decreasing DO*//? at constant E*. Validity of the Graphical T.P.Di. Treatment The linear relation between lg(T&//?) and l/TM stemming from a mathematical treatment of eqn (1) in its approximate form in which all sum terms with n > 1 have been neglected, was previously used to yield E and Do/$ values from experimental TM values at different /3 values.' Fairly good linearity was usually obtained and, assuming peaks to be symmetrical, E was shown to be independent of the number of sum terms included in the calculation (implying that this linearity is practically conserved in the total curve).However, no real attempt has been made to validate this behaviour systematically over a wide range of E, Do and /? values. The computation of the t.p.di. curve from eqn (8)D. Fraenkel and A . Levy 1825 Table 2. E* and D,* values recalculated by graphical method computation parameters recalculated parameters ~ ~~ E* D,* E* error (%) gD,* D,*(g=1.5) ~~ 4000 10' 8000 lo3 1 os 16000 lo6 1 0l6 32000 1014 104 1024 3 992 4 049 8214 7 984 16351 16 006 32 703 32 703 -0.2 1.60 x lo1 1 . 1 x lo1 2.7 2.26 x lo3 1.5 x lo3 -0.2 1.35 x lo8 0.9 x 108 2.2 2.15 x lo6 1 . 4 ~ lo6 0.04 1.32 x 10l6 0.9 x 10l6 1.2 i.8ox 104 1 . 2 ~ 104 2.2 3.2ox 1014 2.1 x 1014 2.2 4.84x 1024 3.2 x 1024 now enables such examination, and, as demonstrated in fig.5, lg(T$/B) is indeed proportional to l/TM, at least between 0.02 and 0.64 K s-', and for the E* - D,* combinations of table 1. E* and D,* were recalculated in each case from the slope and intercept of the straight line, respectively, employing eqn (19) in its logarithmic form. The values obtained are listed in table 2. E* values show excellent agreement with those used for the computation, and so are D,* values corresponding to g = 1.5. Peak-shape Expressions Table 3 presents first-term relations obtained from computed curves. As seen, in a rather wide range of E* and D,* values 4 is constant and equals 0.815 f0.015. The assumption that T;+T;- z pT2 is also confirmed with p = 0.996 0.004. Likewise, the assumption that AT; = qwl is shown to be a reasonable approximation, q being equal to 0.813 f 0.01 5.pk;/g = 1.00 f 0.05 and thus, within 5 % error, Recalculated E*(E'*) values using eqn (21) agree quite well with the respective values taken for the computation. The error is < 4 YO, except for E* = 4000, lg D,* = 1 and 2, for which it is slightly larger (ca. 6 YO). However, in these cases Tk/E'* 2 0.1 and the approximation of eqn (5) is unsatisfactory ; even then the deviations of calculated factors from their average values are not too large (see also table 4). Eqn (21) resembles the peak-shape equation developed by Chen12 [given in this reference as eqn (2.1 l)] for the thermal-glow method (e.g. thermoluminescence). However, Chen arrived at the above form in a rather intuitive manner, assuming (after Lushchik13) that the area of the right-hand part of the peak (T > TM) is equal to that of a triangle having the same height and halfwidth, and that this assumption can be extended to the total glow area (whose halfwidth is 0).Kissinger,14 in deriving his 'shape index ' for determining decomposition reaction orders, calculated inflection points of temperature-programmed decomposition curves by differentiation and arrived at a solution similar to ours {eqn (1 8) in ref. (14), with n = 11. However, the starting equation was different than the diffusion equation of the present study. Furthermore, no peak- shape expression such as eqn (21) was derived from the pseudo-quadratic equation obtained [eqn (17) in ref. (14)]. Dk* derived from the roots of the pseudo-quadratic equation [eqn (12)] has a mathematical form consistent with the ordinary t .p.di.equation.' The two expressionsTable 3. First-sum-term relations from computed curves (at B = 0.16) yb (9-4y)i q P q F,p/q ply; 2.3T:/colc 4000 8 000 16 000 32 000 1 2 3 4 3 4 6 8 6 8 12 16 14 17 20 24 488.2 434.0 541.4 395.9 360.2 431.3 331.8 306.5 356.9 284.7 265.9 303.2 632.3 586.3 677.9 545.7 511.2 579.9 426.2 405.0 447.3 348.3 334.0 362.4 824.4 785.0 864.3 677.7 650.7 704.5 496.9 482.3 511.4 390.9 381.8 399.9 860.0 838.1 881.8 730.7 714.9 746.6 634.7 622.7 646.7 539.6 530.9 548.3 107.4 71.1 50.4 37.3 91.7 68.7 42.3 28.4 79.3 53.8 29.2 18.1 43.8 31.7 23.9 17.3 128.9 85.3 60.8 45.4 111.0 83.6 51.9 35.0 97.4 66.5 36.2 22.6 54.6 39.6 30.1 21.7 0.24 0.756 15 2.4445 0.7992 0.9858 0.8326 0.94 1.13 0.20 0.802 12 2.4065 0.8030 0.9912 0.8330 0.95 1.11 0.16 0.834 15 2.3798 0.8060 0.9936 0.8283 0.97 1.09 0.14 0.85772 2.3599 0.8089 0.9946 0.8229 0.98 1.07 0.16 0.841 97 2.3732 0.8070 0.9941 0.8259 0.97 1.08 0.14 0.86361 2.3549 0.8096 0.9955 0.8219 0.98 1.07 0.10 0.89346 2.3294 0.8135 0.9973 0.8151 0.99 1.05 0.09 0.91295 2.3126 0.8172 0.9978 0.8113 1.00 1.04 0.10 0.89692 2.3264 0.8144 0.9973 0.8143 1.00 1.05 0.09 0.91530 2.3106 0.8177 0.9981 0.8098 1.01 1.04 0.06 0.93789 2.2909 0.8219 0.9989 0.9060 1.02 1.03 0.05 0.951 14 2.2793 0.8246 0.9992 0.8004 1.03 1.02 0.05 0.94625 2.2836 0.8236 0.9992 0.8019 1.02 1.03 0.04 0.95433 2.2765 0.8252 0.9996 0.7998 1.03 1.02 0.04 0.96033 2.2713 0.8265 0.9996 0.7965 1.04 1.02 0.03 0.96627 2.2660 0.8278 0.9997 0.7973 1.04 1.02 av.0.815 av. 0.996 av. 0.813 av. 1.00 f0.015 k0.004 f0.015 k0.05 4 250 4230 2 4160 *tr 4110 2 av. 4190 8280 8190 $ 8 050 7970 1 av. 8 120 16100 5 15900 15700 2 15500 5. av. 15800 x. s- 31 100 31000 30800 2 30800 av. 30900 a A: = 2Ti/E* * y = y... = 1 -(T!L T?-)/E+ “Rounded to last three digits.D. Fraenkel and A . Levy 1827 Table 4. Effect of activation parameters on different factors (at /? = 0.16) 4 000 1 2 3 4 8 000 3 4 6 8 16000 6 8 12 16 32000 14 17 20 24 0.9708 0.9965 0.9996 0.9999 0.9995 0.9997 1 .oooo 0.9998 0.9999 0.9999 0.9996 1 .oooo 1.0003 1.0002 1 .oooo 1.0004 0.6780 0.6685 0.6646 0.6648 0.665 1 0.6642 0.6637 0.6634 0.6639 0.6643 0.6637 0.6654 0.6663 0.6657 0.6681 0.6672 av. 0.666 f 0.003 3.53 3.51 3.53 3.54 3.53 3.54 3.56 3.58 3.56 3.58 3.59 3.61 3.60 3.61 3.61 3.6 1 av.3.57 f. 0.04 1.64 1.59 1.53 1.49 1.52 1.48 1.43 1.40 1.42 1.39 1.36 1.34 1.35 I .33 1.33 1.32 av. 1.43 kO.10 become identical at p l y ; = 1. In table 3 ply; values of selected cases are listed and are shown to be indeed quite close to unity. The small deviations from unity are attributed to the approximate nature of the treatment leading to eqn (18). Thus D:* should preferably be calculated from the t.p.di. equation based on the second rather than the third derivative of U, and in terms of o', it is given by the expression which is analogous to Chen's eqn (2.12),12 or in terms of o and assuming El* = P, by the expression If D:* = D,*, then from eqn (20) and (23) we obtain In going from the first-term-based expressions [eqn (15) and (18)] to those related to the whole sum of the diffusion equation [eqn (16) and (20)] one has to determine the factors f and g and examine their constancy over a wide range of activation parameters.This was done using computed TL, TM and o values and the results are given in table 4. As seen,fis almost independent of E* and D,* and equals 3.57 +_ 0.04 (in fact, it grows slightly with D,*/8); from this value, g was calculated using eqn (24). Although it was found to decrease with D,*/B, g can also be considered constant, and is equal to 1.43 & 0.10; this is about the same value as obtained from the graphical method (i.e. 1.5). Thus the final forms of the peak-shape expressions of t.p.di. are1828 Temperature-programmed Diflusion in Zeolites (26) 2.5p and D,* = - exp (3.6TJu).u Substituting eqn (25) into (26) to cancel u we return to the usual form [eqn (19) with g = 1.41 from which E* and D,* can be extracted graphically. It is noteworthy that the value of g obtained in this work is not substantially different from that obtained previously (i.e. 2.4) on treating t.p.di. curves as if they were syrnmetri~al.~ The quantity Di*calcJD,* (table 4) is a measure of the accuracy of the program, since by definition it should be unity. As seen, for A < 0.2 the accuracy is better than f0.05 O h . Finally, table 4 shows that the half-height width ratio, w’/w, is constant and equals 2/3. Comparison between the Peak-shape Method and the Graphical Method The graphical method (g.m.) based on eqn (27) has severe disadvantages.First, it needs ample experimental work since at least three different heating rates, but favourably more, are required for obtaining a single value of E* (and 0:). Secondly, to get reliable values for the activation parameters, a rather broad range of values should be employed. In practice, this range is normally rather limited since at low P (< 0.08 K s-l) instrumental deviations from setting values cause high relative errors, whereas at high /3 (> 0.25 K s-l) time lags owing to poor response of measuring devices (thermocouple, recorder and manometer) introduce increasingly large error in TM. The result of both effects is usually a substantial deviation from linearity of lg(T&/m us. 1/T, plots. Furthermore, we have found that thorough correction and calibration procedures cannot entirely remove inaccuracies in T,, and reproducibility is in many cases unsatisfactory.Since at low E* values (4000-8000) the difference in T, between highest and lowest P would normally be around ca. 40 K, whereas at high E* (> 10000) it usually would not exceed 20 K, the graphical treatment is doomed to produce 10-20% error in small E* values and ca. 50% in large E* values. Nevertheless, this method is acceptable when a general trend is examined (e.g. change in the activation parameters in zeolitic encapsulation of a non-polar gas as a function of a systematic change in the zeolite such as the degree of exchange with a blocking cation). It is especially useful in the case of ‘impure’ peaks (e.g. when diffusion occurs simultaneously in two or more stages, one being the major).The peak-shape method (p.s.m.) is based on the half-height width of the t.p.di. curve, u instead of the change in heating rate, p. Its main advantages are the following. (1) Only one experiment is required. (2) The activation parameters are obtained instantly from the t.p.di. curve. (In the case of integral’curves, o is measured as the distance between the points of half-maximal tangents). (3) One can employ the most convenient and accurately measurable heating rate (e.g. 0.16 K s-l, which is intermediate in typical heating rate ranges). (4) This method is insensitive to small inevitable errors in TM (a few K). The p.s.m, however, is very sensitive to peak impurities, and therefore should be limited to single-type diffusion processes and avoided in cases of overlapping t.p.di.peaks unless reliable deconvolution is feasible. In a sense, the above two methods can be considered complementary to each other. As mentioned above, their applicability in zeolitic encapsulation is shown and discussed in the succeeding paper.D. Fraenkel and A . Levy 1829 1 " " I " " l " " - 0.015 - TIK Fig. 6. Comparison between theory (line) and experiment (points) for the case of hydrogen decapsulation at p 0.1667 K s-' from (a) Na,,-A, (b) Rb,,,-A and (c) Cs,,,-A. The theoretical curves were computed from activation parameters obtained in the experiment, using p.s.m. : (a) E* = 15129, D,* = 1 . 0 6 ~ lo1,; (b) E* = 5941, D,* = 2.1 x lo3; (c) E* = 7675, D,* = 1.13 x lo4.Experimental conditions and procedure are described in ref. (1 5). Agreement between Experimental and Computed T.P.Di. Curves E,xperimental resultsg, l5 of H, decapsulation from Na,,-A, Rb,.,-A and Cs5.,-A are compared in fig. 6(aHc), respectively with computed t.p.di. curves. The above zeolite-gas systems are believed to exhibit a single diffusion stageg9l6 and hence to produce ' pure ' curves. In Na,,-A the diffusion process is P-cage decapsulation, while decapsulation from Rb,.,-A and Cs,,,-A is believed to involve only a-cage~.~~ l6 As shown in fig. 6, the fit between experimental points and computed curves is good. The scatter of points reflects the difficulty in extracting very accurate dU/dT values experimentally. Curves based on p.s.m.-derived activation parameters seem to agree with experiment slightly better than those obtained from g.m.-derived parameters (not shown).In general, fig. 6 supports the proposed diffusion model in the case of zeolitic encapsulation, as well as the mathematical treatment which serves to link experimental variables (i.e. /I) and measurable quantities (i.e. U, dU/dt, w and 7J with the activation parameters E and Do.1830 Temperature-programmed Diflusion in Zeolites Fig. "'""F 0 -00 0.000 300 400 500 600 700 T/K 0.100 m j 0 - 0 1 2 5 0.01 00 I I " " I " " I ' " ' I " " I " " ~ : ( b ) - 0.0125 0- 075 0.0075 20-0050 0.025: y0.0025 0.050 - 0-0.0251 / I\ 40.0025 0.000 0-0000 400 450 500 550 600 650 700 7. Particle-size effect. (-) sum of ten 10 YO sections; (----) curve based on mean value Y,.(a) E* = 8000, D,* = lo4, p = 0.16; (b) E* = 16000, D,* = lo'', /? = 0.16. t * 0- * O 9 a of wt 5% of particles smaller than ro Fig. 8. Change in D,*/D,*,,,, and its maximal error (for s = 20) with cumulative wt Yo. Particle-size Effect An inevitable disagreement between theory and experiment originates from the basic assumption of particle uniformity, which in the case of zeolites is clearly false. In a typical zeolite A sample the distribution of ro (or a, the edge length of rectangular particles) is such that the change in ri (and Do/r:) between the smallest and largest 10 wt % is ca. an order of magnitude. It is therefore legitimate to negate the particle uniformity assumption'. 1 7 9 l8 and prefer, instead, to introduce the particle-size distribution into the computation.This, however, is a rather complicated procedure. Another approach is to challenge the proposed model by comparing curves and activation parameters, obtained as the sum of contributions from different equal-weight sections in a certain cut of a typical size distribution, with those obtained from ro averaged over the whole size range. Such a comparison is given in fig. 7 for two representative cases, the particle-size distribution being that of Chan and Anderson,' which typifies zeolite A. The sum includes ten curves corresponding to ten equal wt% fractions (i.e. s = lo), each characterized by a constant ro value taken as the median. As seen in fig. 7, the agreement between the two curves is quite satisfactory for the two casesD. Fraenkel and A .Levy 1831 1.15 I I I I 8 1.10 - 1.05 - 1.00- 0.95 , 5 10 15 20 S Fig. 9. Effect of sectioning of particle size distribution on the ratio between introduced (int) and recalculated (rec) E*. = 0.16 K s-l. Introduced E* and D,* values, respectively, are: A, 4000, lo3; 0, 8000, lo3; 0, 8000, lo4; ., 8000, lo8; 0, 16000, 10"; A, 32000, lo". chosen. The sum curve is, however, less skewed and exhibits larger w and smaller (d U/dT),,, and TM values. The Chan and Anderson size distribution, like others known for zeolite A,l79l8 obeys the log-probability relation (ie. gives a straight line on log- probability paper). When plotted as D,*/D,*,,v[ = (ro,av/ro)2] against cumulative wt % it shows the behaviour as demonstrated in fig. 8. When the whole size range is cut into twenty equal (5 YO) sections (i.e.s = 20), median Y, values give maximal error (defined as the r-elative difference between the median and the farthest Y, value in a given section) as described by the maximal-error line in fig. 8. Ca. 70% of the sample lies in the range of < 10% maximal error, but lowest and highest 10-15% fractions exhibit larger maximal errors. This means that close to the extremes average ro (and 0:) values are not good representatives of even very small sections. Nevertheless, in a 20-section cut (s = 20) average ro values give an acceptable precision overall. The effect of sectioning is shown in fig. 9 and table 5. For two typical sets of activation parameters the ratio between introduced E* and E* recalculated from eqn (16) withf= 3.57 grows steeply at low s value, then levels off at s z 10.With different E* - D,* sets over a wide range, this ratio at sufficiently large s (say, 20) equals 1 . 1 1 f 0.03. We can regard this as a compensation factor, h, so that when particle size effect is considered and assuming g = 2.9 1.4p D,* = - exp (4.0TM/co). co To test the obtained peak-shape expressions as given in eqn (28) and (29) we have used the results of Chan and Anderson for decapsulation of various gases from a K-A zeolite.* Peak data and recalculated activation parameters are presented in table 6; a comparison is provided with E and Do values as calculated by Chan and Anderson using a complicated and lengthy procedure. As seen, the agreement between the two calculation methods is very good, emphasizing the advantage and effectiveness of the1832 Temperature-programmed Difusion in Zeolites Table 5.Effect of size sectioning on peak data and recalculated E* _ _ _ _ _ ~ parameters introduced values obtained ~~ E* lg D,* s (dU/dT),,,/ lop3 K-l TM/K W l K EL 4 000 3 20 8 000 3 20 4 2 3 4 5 7 10 20 8 20 16000 10 2 3 4 5 7 10 20 32000 17 20 9.53 5.20 7.1 5 7.03 7.00 6.94 6.89 6.85 6.80 15.65 11.95 11.73 11.6 11.5 11.4 11.35 11.30 13.4 314.5 598.3 525.9 525.5 524.1 523.1 522.8 522.6 521.9 338.1 563.9 562.9 562.4 561.9 561.2 561.0 561.5 720.8 95.5 175.0 127.6 129.6 130.7 131.1 131.7 132.8 132.9 58.1 74.92 76.73 77.82 78.14 78.53 78.85 79.48 65.55 3 697 7 302 7 738 7 607 7 503 7 450 7 403 7 342 7317 7 024 15 152 14742 14510 14425 14317 I4 249 14 161 28 296 Table 6.Analysis of results of Chan and Anderson (CA)" for decapsulation from K-A - gas P/K s-l 0.2833 0.2095 0.1 167 0.0888 N2 Ar 0.2805 0.222 0.181 CO, 0.283 0.233 0.183 0.1 14 T,/K o / K E/kcal mol-lb Do x 105/cm2 s-lC 616 144.6 603 168.5 578 158.7 563 164.8 CA calculation : 630 159.1 620 176.4 614 188.0 CA calculation : 512 153.8 500 140.0 490 146.7 467 138.8 CA calculation : 21 .o 17.3 16.8 15.4 av. 17.6 17.3 20.0 17.4 16.0 av. 17.8 15.8 13.6 14.3 13.1 12.6 av. 13.4 13.0 21.38 0.89 0.68 0.20 av. 1.27 2.174 5.794 0.697 0.197 av. 0.796 0.303 0.485 1:157 0.344 0.250 av. 0.469 0.143 a Ref. (8) E = 8 x 10-3T2,/~. Average values, DO,av = exp C (In Doi/i) . (1 )D. Fraenkel and A . Levy 1833 simple and handy peak-shape approach. Since the particle size effect is reflected in the reduction of E* by only ca.lo%, for many practical cases in which experimental inaccuracy is in the same or larger magnitude this effect can be ignored. Conclusions Computed t.p.di. curves, based on the solution of the theoretical diffusion equation for uniform spherical particles, fit fairly well with experimental curves obtained in decapsulation of H, from different cationic forms of zeolite A. For the calculation of the activation parameters E and Do, either the ‘classical’ t.p.di. equation [eqn (19)] or peak- shape expressions [eqn (16) and (20)] can be employed. The factors g in eqn (19) andfin eqn (16) equal 1 and 2.3, respectively, for ordinary t.p.d.; they have been found to be constant and to equal 1.43 and 3.57, respectively, in t.p.di.which basically differs from t.p.d. in having a sum of exponents rather than a single-exponential term in the relevant rate equation. The above values were computed for a wide range of E and Do. The graphical method employing the ‘classical ’ equation for a series of experiments made at different heating rates, and the peak-shape method based on the half-height width of the t.p.di. peak, complement each other; the latter method is simpler, faster and usually more accurate, but the former method is better in cases of impure peaks. Accounting for particle non-uniformity the factors in the peak-shape expressions have to be corrected. The modified expressions obtained show excellent agreement with literature results based on a treatment in which a particle size distribution function was incorporated into the t.p.di.equation (table 6). T.p.di. is a powerful tool in studying the kinetics of gas diffusion in zeolites. Employing its peak-shape expressions makes the derivation of the activation parameters for diffusion much easier and usually more reliable than before. Glossary decapsulation heating rate (t.p.di.) temperature of maximal rate of release of encapsulated gas during t.p.di. half-height width of differential t.p.di. curve (i.e. dU/dT us. T ) decapsulation time (t.p.di.) decapsulation temperature (t.p.di.) fraction of (encapsulated) gas diffused out at time t (temperature T ) gas constant activation energy for diffusion radius of zeolite particle (assuming uniformity) diffusion coefficient pre-exponential factor of the diffusion coefficient diffusion coefficient at the inflection point of the (differential) t.p.di. curve temperature at the inflection point of the (differential) t.p.di. curve number of equal-weight sections of a particle-size distribution particle-size compensation factor in peak-shape expression superscript denoting restriction to first sum-term in the diffusion equation [eqn (1) with n = 11. References 1 Y. Amenomiya, Chemtech, 1976, 6, 128. 2 J. A. Konvalinka, J. J. F. Scholten and J. C. Rasser, J. Catal., 1977, 48, 365. 3 A. Brenner and D. A. Hucul, J. Catal., 1979, 56, 134. 4 J. L. Falconer and R. J. Madix, J. Catal., 1977, 48, 262. 5 J. S. Rieck and A. T. Bell, J. Catal., 1984, 85, 143.1834 Temperature-programmed Diflision in Zeolites 6 D. M. Jones and G. L. Griffin, J. Catal., 1983, 80, 40. 7 R. J. CvetanoviC and Y. Amenomiya, A&. Catal., 1967, 17, 103. 8 Y-C. Chan and R. B. Anderson, J. Catal., 1977, 50, 319. 9 D. Fraenkel, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2029. 10 D. Fraenkel, B. Ittah and M. Levy, J. Chem. SOC., Faraday Trans. 1, 1988, 84, 1835. 11 J. Haake, J. Opt. SOC. Am., 1957, 47, 649. 12 R. Chen, J. Appl. Phys., 1969, 40, 570. 13 C. B. Lushchik, Dokl. Akad. Nauk SSSR, 1955, 101, 641. 14 H. E. Kissinger, Anal. Chem., 1957, 29, 1702. 15 D. Fraenkel, R. Lazar and J. Shabtai, in Alternative Energy Sources, ed. T. N. Veziroglu (Hemisphere, 16 D. Fraenkel, J. Chem. Soc., Faraday Trans. I , 1981, 77, 2041. 17 J. D. Eagan and R. B. Anderson, J. Colloid Interface Sci., 1975, 50, 419. 18 D. M. Ruthven and K. F. Loughlin, Chem. Eng. Sci., 1971, 26, 577. Washington, D.C., 1978), vol. 8, pp. 3771-3802. Paper 71631 ; Received 9th April, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401817

出版商:RSC    

年代:1988

数据来源: RSC